I agree with others, opentable is overall a convenient tool, but most restaurants hold back some reservations. I've many times found lack of availabilty at places I want to go, but a phone call reveals that I can book.

One thing I've noticed with a few places here in Los Angeles, the restaurant will only take reservations on opentable for half past and on the hour, but a phone call can book a table at :15 and :45.