A few observations from my trip:
I flew HNL-MAJ-KSA-PNI-TKK-GUM, stopping en route in MAJ, PNI and TKK.
MAJ:
I spent 48 h in the Marshall Islands, arriving on UA 154 at 10:35am and leaving at 11:30am two days later, which gave me two half-days and a full day in between. I stayed at the Robert Reimers Hotel (RRH), which in my view is the nicer one of (effectively) the two hotels on the island, the other one being Marshal Islands Resort (MIR). It also has faster WiFi ($15 per day) than MIR and I liked the Tide Table restaurant at RRH more than the one at MIR (which is generally consistent with the majority of reviews I've read). Having said that, both hotels are very dated and neither restaurant is anything to write home about. I absolutely loved the fisherman's breakfast at Tide Table though (tuna steak, overeasy eggs, steamed rice). Tuna with rice is generally the staple along the Island Hopper's route and you'll see it in various forms (steak, sashimi, poke) during your trip. Needless to say, it's fresh, it's delicious, and a safe choice.
Both RRH and MIR were relatively responsive over email. Both have free pick-ups/drop-offs at the airport.
I rented a scooter from MJCC, the Japanese hardware store which is halfway between RRH and MIR. The cost is $20 per day plus $100 deposit. While taxis are cheap ($1 a ride), I loved the freedom the scooter gave me. The first day, I went all the way west to Laura beach, stopping at the WWII memorial on the way; that's around around 25 or 30 miles and it took me a little over an hour to reach Laura. The island becomes more green and lush past the airport, and it's a very nice ride. You can see the water on both sides during large part of the trip. Laura beach was slightly disappointing, but still a nice place to hang out for a bit and I recommend heading there.
On the following day, I went diving; there's currently only one operator on the island, Raycrew, run by a Japanese guy named Hiro. There was no one else diving that day so I had to pay x2. I wanted to go to the Arno Atoll as I heard good stuff about it, but it was too choppy, so we were diving in the lagoon - two dives, with a plane, helicopter, cruise ship and some coral - overall quite nice and Hiro is a super friendly guy with good stories. Note that the largest wetsuit he has is L (and it's a smallish L - I'd say for a slim 6ft person). Water in the lagoon is warm though.
In the afternoon, I took a boat from RRH to Eneko island, which is one of the islets a bit further up in the lagoon. It was okay, with some nice views on the way. I'd definitely do it again. They may have boats to other places - inquire at the reception.
MAJ airport has decent WiFi, but you can only use it for 30 minutes once in 24 hours, so use it wisely
You can of course do 30 minutes on your phone, then 30 minutes on your iPad, etc.
PNI:
I had around 24 hours in PNI. Wish I could do 2 days, to do some extra hiking and/or diving, but could't make it work given the UA schedule. Note that Air Nauru and Air Niugini also operate the PNI-TKK sector - I was thinking about calling UA to drop the PNI-TKK sector and flying PX instead, which would give me an extra day and a half on the island, but would mean a day less of diving in Chuuk. I eventually decided to stick to the original schedule, but it's worth knowing that this option exists; I've flown PX before (from HIR to POM) and it was perfectly okay, although obviously they had a recent mishap in TKK.
Surf Club (Aussie-run, I believe) is the nicest place on the island and also has the best restaurant. It was full so I stayed at 7 Stars Inn instead. Very basic, but immaculately clean and super nice and helpful staff. They helped me arrange a car (newish Hyundai Tuscon, $65 for 24 hours from arrival until departure the next afternoon; I picked it up at the hotel (but could've picked it up at the airport) and dropped it off the airport the next day). I also saw Budget at the airport, which seemed to be the only international chain.
I wanted to arrange a kayak and get it to Nan Madol but that proved to be difficult as it was getting late and you basically need to transport the kayak to the other end of the island; it was low tide the next morning, so kayaking wouldn't work. 7 Stars can put you in touch with a guide (Shawn) - he was unfortunately unresponsive to emails when I tried to arrange things in advance, but now I know I could've chased 7 Stars Inn as they seem to work with him a lot. I guess if it's all arranged beforehand and the tides work in your favor, you can try to head to Nan Madol with a kayak straight from the airport when you land. On the other hand, visiting on foot was fine as well - I didn't feel like I was missing anything. Of course it's hard to visit all 92 islets on foot, but wading through the canals at low tide is doable and easy. I was the only person in the ruins, and as others said, they're spectacular. You can 30-45 minutes to see the main structures; I spent around 2 hours there. These are by far the most spectacular ruins in the Pacific (not much else really, I've seen some stuff in Tonga and Wallis & Futuna). You'll need some change for fees. There's also Kepirohi Waterfall just before you hit the fork in the road leading to Nan Madol - it's a short 5-minute walk to the most beautiful waterfall on the island. You can Nan Madol and Kepirohi as a boat trip (organized by Surf Club).
I also did the Sokeh Ridge (more of a walk than a hike), with some very nice views of Kolonia and WWII guns etc scattered around. Sokeh Rock is a bit harder, I think you definitely need a local guide as there are no signs. One tip - download offline maps on Google Maps for each of the islands you're visiting - makes life much easier.
I mentioned the Surf Club restaurant earlier, which is a great spot for dinner. Lovely location in the marina, good sashimi, overall nice vibe. For lunch, I went to the restaurant at the Joy Hotel that a few people recommended to me - they have seafood bento sets for lunch which are delicious and great value.
I also did a quick trip to Palikir - the capital, 15 minutes from Kolonia - before heading back to the airport.
TKK:
I spent 4 and a half days in Chuuk. I stayed at the Blue Lagoon; the other options are Truk Stop and Level 5. The latter is a compound 2 minutes from the airport, not very appealing (but has the only espresso machine on the island). Truk Stop didn't win my heart over either - went there for dinner one night, wasn't a huge fan of the place/vibe, and there wasn't really anything on the menu which I couldn't get at the Blue Lagoon - except for pizza. WiFi seemed to be faster than at the BL. Taxis stop running around 5pm and arranging transport is surprisingly difficult - you need to let BL know in advance. Truk Stop will drop you off after dinner. Otherwise it's a 40 minute walk, but I've heard mixed stories about safety on the island.
Blue Lagoon - as others have said (and as you can read on TripAdvisor), very very dated. Rooms are basic. WiFi is painfully slow (seemed to be better around 4-5am though!). Food is okay - there's a bit of a variety and the menu is fairly expansive by Pacific standards, but if you want something that is fresh and tasty, it's really whatever fish is fresh that day. Usually, it's tuna, and sometimes reef fish.
The resort itself is lovely though - the grounds are absolutely beautiful, a bit like living on a copra plantation. The sunset bar is a great place to hang out (and I found out on the last day you can actually order anything from the restaurant's menu there, which is a much nice alternative to eating in a fairly nondescript and run-down restaurant). The bar has a good selection of beers (including Big Wave and Island Lager) and has it's own WiFi (you need to ask for the password). There are two hammocks nearby, the closer one is within the WiFi range - a much nicer place to use the internet than the depressing hotel lobby. I generally concur with the TA reviews - I'd give Blue Lagoon 2 stars out of 5. Would I stay there again? Absolutely yes!!
The diving operation is okay. The diving itself is spectacular. There are 2 morning dives and then 1 afternoon dive - I did a total of 9 dives over 3 days. With the number of wrecks in the lagoon, you can easily spend a week diving and won't be bored. As you will read in the reviews on TA, the Dive Shop is okay, but the operation is a bit chaotic, guides' English is not great, everyone pretty much jumps into the water without any briefing etc. There are some deeper wrecks (like San Francisco Maru), but a lot of them are relatively shallow and easily within AOWD or even OWD range. Not a place for beginners (despite what the website says), but if you're not a total newbie, have decent buoyancy and let's say 20 dives under your belt, you'll be totally fine. The dive shop has EANx 30%.
It seemed that many people continue from TKK to Yap and/or Palau - I'd love to do that but couldn't on this trip due to time constraints.
On the way out, my flight was delayed by 19 hours, which the hotel staff said is nothing unusual.
As per the UA website, the check-in cut-off for all Micronesian islands is 90 minutes. BL insists though that it's 3 hours, no matter what you tell (or show) them, so the shuttle leaves the hotel almost 4 hours before the flight, which means you'll just be sitting at the airport for 3h+. I'd just take a taxi and go a little later.
Overall...
All places that I have visited were great. I was positively surprised by the Marshall Islands as based on some trip reports it didn't seem like there's much to see there. 48 hours was a good amount of time to spend on the ground. I wish I had more time in PNI and stopping over in KSA would also be great. I was surprised by how lush and pretty the interior of Pohnpei was (it reminded me of Samoa), and everyone seems to be saying that Kosrae is even more lush and pretty than Pohnpei. Also, a number of people recommended diving and Pohnpei and Kosrae, saying it's worth doing it even if you're heading to Chuuk next.
Would I do this trip in one go, without stopping in MAJ, PNI and TKK? Absolutely not. As much as I like to fly, it's just tiring, the views are okay but not spectacular (compared to some South Pacific islands I've been to, e.g. Tuvalu, Solomon Islands or Samoa).
United itself - I found both the soft and hard product shockingly bad (I did the whole trip in J). Obviously, I didn't expect the product to be on par with Asian or European airlines, but I felt like it was significantly lagging behind even DL or AA. The new "reimagined app" - well, 5 days after the trip it was still showing that I'm onboard the TKK-GUM flight. The 19-hour delay where UA thought $10 food voucher is appropriate for the length of the delay, and a lost bag on one of the sectors nicely complemented the overall experience.
One last side note - I was using the Lonely Planet South Pacific & Micronesia guide, which was published in 2006, so it's very dated, but I still found it very useful - things on Pacific islands clearly don't change overnight (the newer LP guides have dropped Micronesia and cover the South Pacific only). Also, note that since the islands are quite accessible from HNL/GUM and since they were part of the Trust territories, they're quite westernized and there's a fair amount of tourism so you won't have the same experience as e.g. in Tuvalu or Kiribati - don't expect a tropical paradise, but it's still a fun trip and great places to visit.