Honestly, I loved the Mondrian from top to bottom. The service was impeccable - everyone was all smiles and very helpful (the pool attendant was terrific btw). The room was very clean and we had a view of the bay with a small balcony (perfect for breakfast and happy hour). Even the food that everyone complained about was more than half decent.
A few things to note: you are on the bay side away from the hustle and bustle of prime South Beach (which was a plus in my opinion, but won't be for everyone). Just a quick walk to the major hotels/restos, etc...
Pool was lovely. Kind of chill...and easy to deal with.
Also, on the plus side, a Whole Foods and Starbucks are within walking distance.
Only really awkward moment (and reason for not giving 5 stars) came at check-in. I booked and paid for a 1 bedroom with balcony. The front desk employee gave me a key and I asked "Is this a one-bedroom?"...she said yes. The room she gave us was not a one-bedroom nor did it have a balcony. I called to fix matters and it was a little awkward, but then we were given the correct room. Not sure if it was a bait and switch, but ah well...it was bliss after that.