I went to New Jersey on a whim and didn't book anywhere to stay. Booking.com was able to find me rooms that came open immediately when not much was available. After we arrived at the hotel the woman said we were lucky because they had a very last minute cancellation and they just opened back up to book it so it worked like a charm for us two nights in a row when finding another hotel. Not problems at all.
On another note, while trying to book in Panama City the rates looked great for a week at a hotel. When checking out, the price was at least $300-$350 more than the advertised price. I understand there are taxes, etc. but just seemed like a huge increase. So I went to the hotel site directly and found a promotion and booked the same exact room, etc. and got it over $350 cheaper. What's nice about Booking.com is you can use PayPal and get 6 months to pay unlike going straight to the source to pay which usually doesn't take PayPal. So, overall, my experience has been okay with Booking and I'd use them again.