We've used it several times and had one glitch, several years ago, were the time had mysteriously changed. When we arrived at the restaurant, we were told that our reservation was for 2 hours earlier. However, they were still able to seat us. When I got back home, I saw where opentable showed us as a no-show. I contacted them and had to proof to them that my requested time was the correct time. Once I did that, they gave me my credit and extra credit for my trouble.
We've used them a couple of times since then without problems and have 2 upcoming reservations with them. One at Tchoup Chop and one with The Palms restaurant at HRH. We booked The Palms reservation because it was one of their 1000 credit choices.