I've been going to Miami for over a decade to play for a long weekend. My first experience at the Mandarin Oriental Miami was a pleasant one. You definitely feel like your in the Caribbean if you have a room facing the ocean or if you're lounging at the pool.
Rooms are huge by the standards in Miami, and each room has a decent size balcony. Unfortunately because of all the black birds the balconies can be used as a restroom by our feathered friends. I suggest the hotel get some hawks to patrol the premise.
Room service was quick, but the food was definitely not worth the money.
The staff was amazing from beginning to end, so if service is your number one priority this is a place for you.
Be aware it's a $30 cab ride to South Beach, so if you're a big Lincoln Road or night club person, you may want to look for a hotel in Collins Ave.