Avoid the Clevelander. It is a marginal, old hotel. The only thing going for it is that it is waterfront, being across the street from the beach. It has a popular but very casual bar--this is a spill beer on your shoes kind of bar. Rooms were old with dated furniture and no carpet. My window shade would not close, and there was a bright light outside my window. Stayed there 2 nights--mgmt never fixed it, never moved me to a different room, or gave me credit on teh bill.
Also avoid the Fountainbleu. Its really not in South Beach--its up around 44th Street. Its an old property and not fully renovated. Numerous complaints in the Hilton Forum.
Ultra high end is the Delano. If you go for the boutique hotels, look for the ones a block off the beach. These are generally in better condition.