My one complaint though-not all openings are on there. They list what the restaurant tells them to. I have gotten "no availability" on the opentable site, then called the restaurant and gotten an opening. So its worth it to call if opentable says its booked.
Plus, opentable has that points program, where you rack up points for gift certificates by making reservations.
I personally like Dinner Broker, but I've used both. The nice thing about DinnerBroker is that they have some discounts with restaurants if you make slightly off-peak reservations, I saved 20% at Lutece using one of their discount reservations and we dined at 5:30 PM, not really that far off peak IMO.
I'll also recommend Open Table. I've had great success at a variety of restaurants through their site on very little notice, and it's certainly easier than cold calling through Zagat. I'm interested in checking out Dinner Broker, though; that off-peak discount is pretty attractive.