Opentable usually works fine for me. I like the $20 dining cheque you get in their reward program.

Many restaurants don't allow Opentable to reserve all their tables, so if the site shows no availability it's worth a phone call.

I recently read a magazine article (can't remember where) talking about Opentable from the restaurant's perspective: apparently many restaurants don't like it because it's expensive and difficult to cancel the service once it's started.