My girlfriend used opentable to make us a reservation for my birthday this year. Only problem--the restaurant was not open that day and opentable didn't know it until about 10 days beforehand, when we suddenly got an e-mail cancelling the reservation without explanation. Had to call the restaurant to find out why. Seems that if they have a direct tie-in to the restaurant's system, they should have know this sooner. Thankfully we managed to get a reservation somewhere else (this was actually a bit of an issue since we were leaving for Mexico for the week prior to my birthday).

Other times we have not had a problem, but this experience caused us to raise our eyebrows a bit.