Stayed here for a business trip recently and had a really great time. The room overlooked the beach and was nice-clean, functional. Not the largest you'll ever find, but when you go to South Beach, you aren't really spending much time in the room, anyway. Most hotel rooms at the other hotels on SB are boutique hotels that are catered to tourists who want to go to the beach, drink until 3 am, and make a lot of noise. You can do that all at the Marriott, but you are also just a few steps from the din, and so can also relax and take it easy and get away from that sometimes overwhelming scene. It also works great for business. I had meetings near the airport, and there's easy access to 95 and there isn't any traffic to speak of (I've stayed elsewhere in Miami itself and taken longer to get to the same place--and not be right on the beach).
The other reviews talk a lot about the great front desk staff. I was always greeted when I came back and was always wished well when I left. The maids actually said hello when I passed in the hall, which is something that doesn't happen that much. Can't say enough about how well I was treated. Didn't have any real problems or concerns, but they still made me feel welcome and important. Great staff.
Location is at the end of SB, a few steps away from all the touristy stuff, but right on the beach. Pool is kinda small, but nice. Didn't try the restaurant, but there is a place called The Big Pink a block away that was great if you like old diner-style food that is very reasonably priced for the area. If you walk away from the main tourist area, then you get to some nicer, more traditional restaurants.
Overall, a great hotel in a great location. Really nice and helpful staff, really good service. I'd stay here again.