I use Opentable a lot (and just noticed I am a VIP! - probably have been for a while - though cannot say I have ever noticed any different treatment). In some cities like Las Vegas, Chicago, or Orlando, I find that most of the restaurants I am interested in (as a business visitor) are on it - and often it is easier to make a reservation online vs. calling. Have earned probably $200+ in dining checks and been able to use them without issue except once (was refused - no big deal - used it a week later).

To add on to nick_marquise's comment - there have been several times where a restaurant had no reservations (either online or on the phone), so I made other plans, but checked back at opentable before leaving - only to find a reservation had opened up at choice #1. Since many of the restaurants are using opentable to maximize their seat utilization, when they get a cancelation it appears almost immediately online (some hold back or have a cutoff time, but most do not). I also use opentable a lot on my mobile phone when we are out and decide we are hungry - great quick snapshot of where we can eat within the next 15 minutes with no wait.