I haven't used it in quite some time, but the times I have used it, it worked great. I used to swear by it as I hate calling around for reservations.

My understanding is that it ties directly into the restaurants reservation system (which is what OpenTable is really selling) so that in theory there shouldn't be any difference between calling the restaurant and using the OpenTable website. That said, it can't hurt to talk directly the restaurant as they may be willing find a way to fit you in.