It's all about the location
Unlike other Grand Hyatts this one isn't going to knock your socks off. But the location couldn't be better. Steps from Grand Central on E 42nd Street - you're well placed to get to practically any destination in Manhattan and beyond.
The lobby is modern, and clearly striving for impressive/artistic. Comes across as a pretentious/bus station. But it does have a handy Market where you can pick up snacks and hot meals 24hrs a day without being ripped off by room service. I was on the 31st floor, and was impressed with my room at first. Good size, quiet, plenty of storage, humungous TV, super comfy bed, decent shower. 
Annoyances: An iron but no ironing board (although one turned up quickly after I requested it). No flat surfaces anywhere in the bathroom to nowhere to put even a toothbrush and toothpaste. Kind of annoying lighting (no switches - all electronic). And a rather unaccommodating approach to late check out. I was willing to pay extra to have a few more hours in the room, but was told that I could have another hour and that was it. Fine if you're busy, but it was a Sunday afternoon... I'd be surprised if they were full.
Hats off to the chambermaid, who was both friendly and helpful and 100% on top of her game.