I stayed at the Gramercy Park before it was renovated and became a design hotel. It was interesting and the key to the park was fun, but my room was horrible and I did not appreciate having to carry luggage down the fire stairs when all elevators were out of order and it was time to leave for the airport.
Even with the changes, I'm not sure I want to go back, although I did have some good meals during that weekend in NYC.

For an interesting design (a semicircle standing on the curved end), São Paulo Design Hotel (that's its name) is potentially fun. It seems to have some lively bars and a rooftop place with good views as its located on the top of a hill or ridge overlooking the park with the art museum and the university campus.