What a great weekend. Trip started out great (upgrade to a Fantastic Suite, we were staying on Starwoods points). While the suite is at the end of the hotel (near the road) and originally worried us a bit, it turned out to be a terrific room. 270 degree views including beach and Miami skyline (which is nicer to look at in the evening than a dark beach). Very quiet room - never heard a peep from the room next to us or outside (which is not the norm for most W's).
We spent 2 days around the pool. Service was perfect. The staff is very friendly, zero attitude and always there when you need a drink. The pool is not huge (e.g., compared to the FountaineBleau pool) but is large enough, tropically landscaped, the water warm and the bar very cool. We rented a cabana one day which was also a nice touch (in particular, if there is some good sports on). Overall, a fun scene where you can definitely enjoy a few days of relaxation. We didn't spend much time on the beach but it also looked very nice (they have food/drink service on beach) and could serve as a nice alternative to the pool for longer stays.
The main bar was always lively with great people watching. Bartenders were great and take pride in their drinks (they were having a drink competition and working on experimental drinks all weekend...). Mr. Chow's is crowded (tables very close) but we were surprised to find we really enjoyed the dinner and would go back (we also went to Casa Tua which was off the charts).
All in all, we'd go back in a heartbeat. Congrats to the W for surpassing all our expectations. We stay at W's frequently but wouldn't call them luxury. However, W South Beach is a totally different experience.