Embarkation: Pretty easy. We were told that any bags that don't fit on the carry-on x-ray machine need to be left with the porter. Note: be aware that any alcohol in your carry-on may be confiscated. Bags made it to room pretty quickly: within an hour or so.
Rooms: We were towards the front of the ship in an exterior room on the lowest level. Our room was only a little chilly, which was easily corrected by closing the ceiling vent (it turns with a dial). Biggest room we've ever had on a cruise. And being on the lowest level (Riviera) in no way made the cruise less fun. Elevators are reasonably fast (each side of hall is independent, so press both up/down buttons as needed), and there are extra available lifts in the concourse.
Dining: We got a 5:45 time, asked for the 6:15 time (we had our 15 month old with us), and didn't get it. No big deal though - it's nice to eat while you can still see the ocean, and there's always more food later! I also recommend you check out the midnight Mexican food buffet on Wednesday night.
Music: Next to the pool, a band played various dance songs (UB40, "Hot Hot Hot", etc). They were decent, although I prefer steel drums on a cruise.
Key West: Trolley takes passengers from ship (on military base) to center of town - our driver pointed a few things out along the way. We toured the Hemmingway House and drank a beer at Sloppy Joe's. I'd recommend both.
Cozumel: We went to the Chankannab park, enjoyed snorkeling and *hot* weather. A guide (included with the snorkel gear) will take a group into the water and bring food to entice the fish. The park is nice, has a lagoon for small children, dolphins, etc. Lots of sand to tan on, but it is not a typical beach in that you walk down steps to get into the water here (other than the shallow lagoon).
At Sea: We did a couples message, which we enjoyed. On the first night, there was a free liquor tasting in the store, which I thought was a cool idea. All-in-all a smooth trip.
Disembark: We didn't want to get up at 6 to leave at the 7:10 am early "take your own baggage" alternative, so we stayed on board until 9, and then waited in a line for about 15-20 mins. No big deal. Our bags were not difficult to find at the port and it was a smooth experience.
Other notes: For parents: You will likely not need a car seat in Cozumel or Key West. Our baby was allowed to ride in our taxi in Cozumel without one (it was a van). For non-US residents: you will not need a visa to visit Mexico (you're there under 24 hours)
Bottom line: Fascination is in fine shape, and having cruised on RCCL and Celebrity, I see little difference between those lines and this experience we had on Carnival. Except that Carnival's rooms were bigger, and you had to pay for soft drinks. I'd sail this ship again in a heartbeat.