I just signed up for it and I think it's great. I used Opentable before and it was way too expensive for what I got in extra business. I have a website with Google Analytics and a lot of the visits are from Urbanspoon, so I checked out Rezbook and signed up. Within a week I got a few reservations, some no-shows (now I call them to confirm). It's really cheap advertising - $2 per person for each seated reservation with no monthly fee. The app is only available on an iPad (I already had one) but you don't really need one if you're just using it as an add-on.
It doesn't give any points so it probably won't work for Opentable point-collectors (who cost me $1.35/person for 6 ppl spending $50 total, main reason I ditched OT), but so far it's been great advertising for minimal $$.