Lovely, unless something goes wrong.
I have stayed at a number of Hyatt's. Enough that we were able to book four nights at the 48 Lex on points. For 15000 points a night it seems like a reasonably good fit. 
We tried to check in at 1:00pm (check in time is at 3:00pm). Our room wasn't ready but we were told that we could hang out in the Lex Lounge (very nice but without complimentary internet which would have been helpful) on the second floor. We stuck around for about half an hour. The room still wasn't ready so we figured we'd best get on with our day. They called just before three to indicate that the room was ready and that our bags had been moved there. So far, everything was fine. Before dinner we dropped in and do some work, unpack, etc. I noticed that the room was rather hot so I turn the air down from 24 to 21. After a long night out we got back to the room and it was still 24 degreeds. I called the front desk and they sent an engineer. It was after midnight and he was still trying to figure out what's going on; we made the most of it by having a glass of wine on the patio under an umbrella in the rain. We were told it was fixed. It wasn't. I had a terrible sleep. 
There are three things I need from a hotel. Internet (which costs $14 a night but they were good enough to comp those charges). Hot water (which came eventually, turn your shower on and wait for a while before jumping in). And, most importantly, a good nights sleep. Night one was a write off; it was too hot and humid for me to sleep.
The next morning I talked to the people at the front desk. I said there were two options, that the AC be fixed or that we move to another room but that I preferred the former as all our cloths were in drawers and we were rather settled in. They said they would call either way and that I need not worry, that "they would take care of me." They didn't follow through with either. We got back to the hotel at 2:00am after a long 18 hour day out in NYC. I called the front desk to find out if the room AC was looked into (no call, no note, nada) and was told there was no record on the file. The second night of sleep in a hot room with a warm comforter was equally miserable. 
Unable to sleep I got up early and requested to talk with the manager. I talked with Jordon, Director of the Front Office. He was helpful and told me he'd contact me soon. Within half an hour we were moving rooms from 406 to 405 as he couldn't guarantee the AC would be fixed. As were were trying to pack up and move from one room to the next an engineer came down to look at the AC. Surely he could have waited until we were out of the room.
We were not charged for either of the first two hot nights and our points were restored. While I recognize these things happen it took two days to resolve the issue and two sleepless nights; that is unacceptable.
The hotel looked nice but there were little issues that added to the frustration. The first terrace didn't have a table. The second had a table but it was covered in bird feces and was never cleaned. The remote in the first room for the entertainment unit didn't work. The entry floor in the first room was sticky. In both rooms, even though the reservation was for two, there was only one robe (and one umbrella). In both rooms the bamboo shelve above the toilet was on a slant and looked like it was falling off the wall. The art's side matting in the first room was falling into the picture blocking the image. In the first room the sliding door wouldn't stay closed, in the second it wouldn't stay open and it was improperly installed so it hit the wall resulting in smashed tiles. We drank the complimentary in-room espresso coffee but the coffee was not refreshed the following day (this happened twice) by house keeping.
While the initial impression was good and the staff were very polite, when I needed high quality customer service I got lip service. When I'm told "we'll take care of you," I don't want to be appeased, I want to be impressed, and at the very least pleased. If you're moving guests from one room to another because the first room was not satisfactory, you had best go over the second room with a fine tooth comb before letting them in.
The final frustration was upon checkout (at 5:00am) I looked at the bill and we were charged for a water from the minibar. We didn't take anything from the minibar. They quickly removed the charge but the fact that it was there in the first place should never have happened. 
In New York we have also stayed at the Hilton New York, both were memorable but it certainly wasn't for the same reasons. Perhaps the Hyatt can sell this property to Joie de Vivre or a similar boutique hotel chain, at the very least they can certainly learn something from them when it comes to impressing their guests.
If everything goes right, you're in for a wonderful stay. The beds are soft. The rooms are beautiful and quite. The staff is polite and friendly. 
If something goes wrong I wouldn't hold your breath. While it may be resolved it could take a few attempts and may leave a bitter taste in your mouth. 
A note to all hotel management. Comping a nights stay is a good start but it's not going to bring me back. Proving that you care about my business (even if I used points) and want it in the future means going above and beyond. It's extraordinary what a complimentary bottle of champagne or a personal note does to leave a good and lasting impression. 
Our time in NYC was short and too much of it was spent dealing with room issues, hotel staff and moving rooms.