I love OpenTable. I've been using them for... well, as long as they've been around really, because my ex worked for them for a few months right after they got going.
I used to use DinnerBroker (I'm my mother's daughter, and an astute bargain hunter) when they had some great restaurants on there, but now it seems that OT has cornered the market.
They currently have a live beta of their site, and it's fantastic. It has an attractive new layout, including some upgraded search and sort features, instant access to reviews from Citysearch, Gayot & AOL CityGuide, and a cool new "Other diners suggest..." option.
You can check out the new OpenTable beta here