The Marriott Osaka is a solid 4-star hotel and much better than the average Marriott (certainly at least than the average US Marriott), thanks to a new-construction setting and well-executed Japanese hospitality. It's a bit out of the way for a stay in Osaka but is located directly above a large train station, which mostly makes up for that limitation. I would recommend it for many types of trip to Osaka.

Check In

In classic Japanese fashion, the hotel has an excellent set of directions with detailed maps and photos explaining how to access it from various starting points in the Tennoji train station complex downstairs. Take a look at the related documents links here: http://www.marriott.com/hotels/maps/travel/osamc-osaka-marriott-miyako-hotel/#directions

As others have mentioned, the elevator situation from the ground floors to the 19th-floor lobby is a bit confusing, but ultimately not too big of a problem as each elevator has lighted screens showing which floors it is available to serve. You do potentially have to push a few different buttons to summon all possible elevators, though. I'm also not sure why the hotel lobby is on the 19th floor while all other hotel facilities start on the 38th floor and up, but I'm sure there's some reason.

In any case we were met by a friendly staff member in the very large lobby as we came out of the ground-level elevators, who asked for our name and quickly recognized my Platinum status and escorted us to the club lounge for sit-down check-in.

Room

Rooms are on the 38th to 53rd floors, with the club, gym, and business center on the 38th floor, so between that and the lack of any other skyscrapers nearby all rooms have great views. Rooms on the north side (below xx11 or so, at least on the 52nd floor where I stayed) face central Osaka and, on a clear day, a backdrop of various mountains, while rooms on the south side face a more sprawling low-rise part of the region. The lobby faces north and the gym faces south if you care to check out the view not found in your room. Our view to the north:

Our room was not huge but was a perfectly acceptable size especially by Japanese standards, and was comfortable enough for two people with luggage. Bed was comfortable and perhaps we were upgraded but I think most if not all of the one-bed rooms at this hotel are advertised as queen-sized, but ours was definitely much bigger than an American queen bed. No desk, as is the trend, but a good-sized table with chairs that would be sufficient to work from if needed. Nespresso machine and tea kettle were included. Minibar prices were generally quite reasonable, including 400 yen for a beer and 1500 yen for a half-bottle of still wine.

Bathroom is a bit oddly split with toilet and tiny sink in one room and main sink, mirror and shower in the other, which seems to be somewhat more common in Japan than elsewhere (it seems to be pretty common at onsens, in my limited experience) so may be a cultural thing. The toilet included a Toto washlet, the version with all the bells and whistles -- but a Japanese-only control panel, so good luck figuring out some of the finer adjustments. (The main functions are labeled with pictograms but there are many buttons which are not, especially if you flip down the main panel to reveal yet more buttons underneath.)

One side note: earlier in this thread I asked a question about the room category of "comfort rooms", and it was determined that they are rooms on the 38th floor with a more Japanese style including wood floors. But, it is also worth noting that the club lounge takes up most of the north side of the 38th floor, and as others have noted, the club lounge and presumably any north-facing 38th floor rooms also look out onto an enclosed garden area and have no view. So if you're hoping for a view of Osaka, comfort rooms are definitely the wrong way to go.

Service

I reached out to the concierge before arrival and they were able to book a few restaurant reservations for us in both Osaka and Kyoto. They also provided good recommendations for lunch places in both cities. Perhaps most impressively, upon check-in we were presented with an envelope containing a recap of our reservations and an A4 sheet for each venue complete with a map to the restaurant, transit recommendations, names and addresses of the restaurant and hotel in both English and Japanese (for a taxi driver), and a picture of the outside of the restaurant as most lacked English signage. All around a very impressive service.

In my experience reservations at the top Osaka and Kyoto restaurants are not quite as competitive as they are at those in Tokyo, but if you want to do any high-end dining it is still worth reaching out to the hotel at least ~6 weeks in advance to give them time to contact restaurants when reservation windows open, usually a month out or in monthlong blocks.

Club lounge

For all the usual details, here's the club access information letter provided at check-in:

The club is large and does get busy, though it was never overcrowded while we were there. Service is fairly hands-on, in that you are escorted to a seat like you would be in a restaurant upon arrival, though the staff will happily take your guidance on where you want to sit if it isn't busy. 

The food assortment was solidly above average, with a large breakfast selection including fresh-made eggs station, and a particularly large evening food spread, one night including a roast beef carving station, which is something I've never seen before even at higher-end hotels. If you're not too picky you could easily make a meal replacement out of it if you wanted to.

Booze selection in the evenings was pretty good and a Japanese whisky was offered, though we were told only one bottle gets put out per night and once it's gone, it's gone (another night it was still available quite late, so it's not clear if that was always true). It's not entirely clear if the booze is meant to be self-service -- sometimes, especially earlier in the evening, there is a server there who will offer to make drinks for you, but that person seems to disappear later in the evening.

Location

The hotel definitely isn't in the center of things, but Tennoji is one of the major rail hubs in Osaka so it's not hard to get around.

The Midosuji subway line is right outside the basement entrance to the hotel and runs up the main spine of the city, serving most likely destination areas. Conveniently, some Midosuji line trains heading north originate at Tennoji (the final terminal of the line is further south), so you can usually easily get a seat on those trains, perhaps except in morning rush hour.

Also particularly useful is the half-hourly Haruka limited express JR train which serves KIX airport heading south and Shin-Osaka and then Kyoto heading north, making Tennoji well-connected to all of those places. The Haruka is covered on the JR Rail Pass. And since it is meant mainly for service to KIX, unlike most Japanese trains it has dedicated luggage storage areas, so it would not be too difficult to take the train even with luggage between the hotel and KIX, or if transferring to or from a hotel in Kyoto.

Be aware though that there are also other ways to get between Osaka and Kyoto that may be more useful, particularly for a sightseeing day trip. We did a day trip to Kyoto and took the Haruka there in the morning, and had dinner reservations in Kyoto so knew we were going to return after the Haruka stops running. There are other JR trains back to Osaka station where you can connect to the subway or Osaka Loop Line if you really wanted to use the JR Rail Pass, but we ended up returning on the Keihan line instead -- it runs conveniently along the river near many tourist destinations in Kyoto, saving us a cab ride or long walk back to Kyoto station from the Shijo Dori area, and the Keihan line plus the Osaka subway only cost around 600 yen for the entire one-way trip, so we did not mind foregoing the free Rail Pass journey for the convenience. The Hankyu line is also an option depending where you're going in Kyoto. Both trains run to central areas of Osaka where you can then catch the subway to Tennoji. Make sure you investigate, or let Hyperdia or Google Maps steer you to, express train options on either line, as the local trains have many stops between Kyoto and Osaka.

As for other destinations in the immediate area, there is a significant amount of shopping and conveniences in the train station below the hotel, including a couple of department stores. There is a full-scale shopping mall located across the street and connected by a pedestrian bridge from the 2nd floor. There is also a large park with a number of attractions including the municipal art museum and the zoo just north of the hotel. 

One other handy thing to know about for a tourist trip to Japan is the Takuhaibin luggage delivery services. We were able to send suitcases between hotels in Tokyo and in Osaka for about 1,500 yen each (less than US$15 currently), which saves both the difficulty of lugging suitcases up and down stairs and train stations and of finding very limited space for suitcases on board most intercity trains. The delivery is next day, so you just have to plan for an overnight bag for the last night at your old hotel or first night at your next hotel. You can bring your bag to the bell desk at the Marriott (or at just about any other high-end hotel in Japan) and they handle everything from there and charge the cost to your room.

A more detailed description can be found here: http://www.japan-guide.com/e/e2278.html But really, that page makes it sound more complicated than we found it to be; the hotel really handled everything, including placing the suitcases directly into our rooms upon arrival. One tip: don't try to roll your suitcase around at all before taking off the plastic wrapper it is placed in, or you may find yourself with shreds of plastic stuck in your suitcase wheels. (Unlike at this Marriott, when the bag arrived at the Ritz-Carlton Tokyo it appeared in the room unwrapped, so I guess that's the extra service that fifth star gets you.)