My wife and I spent a weekend in late August at the Montauk Beach House. The lobby and rooms look as if they are straight out of a Restoration Hardware catalog. Our room was spacious and beautifully appointed with new everything. The staff is friendly and helpful, but definitely staffed a little thin - think ahead if you need anything, as it can take a few minutes to get to you.
You will probably either love or hate the atmosphere, depending on what you're looking for. The pool scene during the day is not rowdy or out of control, but definitely a young crowd with fairly loud music. We wanted to take a nap on a rainy afternoon and it was difficult to fall asleep. Same deal at night, but the action moves from the pool to the bar, which is further away from the guest rooms.
Bottom line: it's a swanky place to stay, but too expensive if you're looking for a laid back or romantic atmosphere. If you're a bit more into the party/pool scene then you will be happy if you dont mind shelling out a fistful of cash