We (two parents and a teenager) only stayed for a one-night pre-cruise, but were very happy with The Betsy Hotel. While the room was a bit on the small side, we expected as much given the vintage of the hotel, so no grounds for disappointment. Everything was sparkling and clean and the staff was very friendly and competent. BLT steak was good and worked to accomodate our needs (exceptionally windy evening, so they moved our reservation from outside to inside with little trouble even though the restaurant was full).
The Betsy is just far enough north of the BOOMING party scene that you can enjoy a peaceful stay, yet stroll easily to the fun.
If we stay in South Beach again, we'll certainly book The Betsy again!