Using Olleh does not require any difficult sign up. You can make a reservation online for what you want. Then when you get to the airport, you get what you reserved. It's very simple.

http://roaming.kt.com/renewal/eng/main.asp

I don't get why you'd need a cellphone plan when a WiFi egg for 8000won/day is a better deal IMO.

I spent 29,900 won for a SIM with 1GB of data (and 3,500 won remaining for calls/texts). I'm here for five days. An Egg at 8,000 won per day would have cost me 40,000 won, plus I would have had the hassle of keeping an extra device charged and/or running out of battery while out on the go. I saved 10,000 won and it's much more convenient. And each extra gig is 15,000 won, and it's pretty hard to burn through 500MB in a day (especially as the phone automatically connects to olleh wifi hotspots whenever possible), so even if you're a heavy data user, the SIM is probably cheaper than even the unlimited Egg.

I suppose if you're only on a weekend mileage run or something, the Egg is a better deal.

Although I will grant that the Egg supports LTE, which apparently the Evergreen SIMs, at least, do not.

For me, I burn through data ridiculously (internet radio) so the egg was a good deal for me. Plus, Instagram, WhatsApp, Skype... I put that egg a run through its paces haha.

Just returned from Seoul. We have T-Mobile Simple Choice, and found it worked well, at least for my wife and myself. Upon landing I got a couple of text messages:

It was a 3G connection, so I was glad that we also rented an Olleh Egg for the duration. The Egg was 5,500 Won/day, and we used it for two weeks. It worked well, provided one keeps it charged.

Our son was unable to get his iPhone to roam in Seoul. Apparently he had not updated his carrier settings in some time (these are updated periodically when you connect your phone to a trusted computer running iTunes), so that may have been the cause.

If I rent the egg and take it to Japan will it work?

No, you can't use it in Japan. Only works on the Olleh network in Seoul.

Japan has their own pocket wi-fi you can rent, though.

Do you remember which simcard companies they were selling?

I believe it was called EG or EG Sim? More info found here.

Yes, EG Sim is the main one. Personally, I still stand by the fact that if you're traveling with many people, the egg is a better deal as you can all share the fee. If you stay a looong time and you're alone, yes, the SIM card might be cheaper (Just to get the card with 1 GB of data costs 30,000W). For me, if I was staying less than six days, I'd stick with egg.

Any guidance what people are seeing in terms of battery life using the Olleh Egg? Some quick googling pull up a range of 4hrs to 6-8hrs to 9+hrs depending on the model.

Also, do they use a micro-USB connection for charging? I've got a couple of battery packs that I'm hoping to use to keep them powered through the day.

What about for a solo / duo traveller, for 4-5 days.
Is the Egg still worth it? Say compared to a single sim card.

An egg is worth it. The cost will be 20-25,000W unlimited use total. That means each person will spend roughly 12,000W each. If you each get your own SIM Card, at the cheapest is 30,000 EACH.

One of those portable battery chargers comes in real handy during a full day of exploring Seoul. Also, if you take a break for coffee try to sit near an electrical outlet to charge up your devices.

I never watched closely enough to see precisely when it died. They rented it to us in a semi-padded foam case which held the egg, the charger, and our paperwork. It was a small case, maybe 6" by 4" by 2". Typically I carried the case in my camera bag, with the egg inside. The battery did die a couple of times, but I would confidently estimate that we were getting 6-8 hours of battery life out of it consistently. I can't recall what model we had.

I can't say with 100% certainty that it was a micro USB port for charging, however IIRC the charger was in two parts: an adapter for wall outlets, and a USB cable. So if your battery accepts a USB cable, you can use the cable that comes with the egg, whatever connection it has on the egg end.