I had a weekend break at the Mandarin Oriental Miami - and I really loved. First of all, the location is just perfect - (far) away from the hustle and bustle of South Beach (which I visited and did not like at all ). The island in which the hotel is located offers great jogging/walking opportunities; hotel has great views over the bay area as well over the skycrapers of downtown. I liked the layout of the rooms. I loved laying by the pool area so much that I never even visited the 'beach' they have in the hotel. Service in the pool is good, although a bit slow at times. Food was great in the hotel - especially breakfast was to die for.
In general, the service was top-notch - to give you an example I was warmly greeted by my name when I was escorted to my breakfast table.
There were some hickups too. Perhaps the biggest downside in the hotel is MO bar - of which I've visited several of other Mandarin Oriental hotels in different continents. It lacked the typical 'MO vibe' and actually was a rather dull lounge area in the lobby. The check-in was also a rather odd - I had booked a car with a driver (Mercedes 500) to pick me up from the airport and then take me to Design District (DD). I was hoping to have a quick check in before continuing to DD. Check in was quick but I was a bit astonished when the clerk - after happily welcomed me - told that my room was downgraded - WHAT! Well I accedpted it as I was in a hurry and as he told me that they have a very nice alternative room in a top floor. I think the reason for downgrading (they also reduced the daily price) was that on Sat. evening the hotel (beach and pool area) hosted a wedding ceremony and it might have been quite lound in the room that I had originally booked (room with a large balcony). In fact I'm sure that I enjoyed more the high-floor room as it had fantastic views.
I would definitely return.
BTW: If you like plane watching this is a hotel for you!