It really depends on the hotel and the timing.
Let me share one of my experience I had a while ago. Once I booked a 4* hotel (Imperial Palace) in Las Vegas several years ago. When I arrived the hotel, it was late almost midnight. And almost all rooms were sold out. According to Priceline's policy, the hotel had to accomodate the customer. So eventually they put me in a penthouse room!
Well this kind of incidence only happened once to me... But it did happen. So the experience through Priceline NYOP is really YMMV from my point of view.