There's no doubting that this is a great, impeccably stylish hotel but don't come here if you're anticipating peace and quiet (arguably that is true of Miami in general!) On our first night people came back in at 4am and kept the party going for a few more hourse: usually we would be annoyed but when you consider what the Mondrian is like and who it is clearly designed for we realised that we were the ones who were probably in the wrong place.
The rooms are very slickly done and actually pretty spacious for Miami. Only half of the bay view rooms have balconies so try and request one of those. Stay clear of rooms ending 06 if you can: you get a bit more space but it is right by the lift/ice machine so you get lots of noise from the corridor. Service was actually very good and helpful, in room wifi was quick (and free) and the pool area looked a great place to lounge off a hangover.
Location is interesting, a short walk to Lincoln Road mall and to South Beach: if you're looking to spend all of your time on the sand you'd be better off staying on Ocean or Collins but this was good for South Beach, Lincoln and downtown, with great bay views.
All in all a great choice if you don't want to stay in a South Beach shoebox but are ready to get dressed up and party hard (and we were also lucky to get a great deal on booking.com). If your tastes are a little more sedate however this may not be the place for you...