I've purchased certificates from Restaurant.com for several years. First in San Francisco and now on Central Oregon Coast. I've never been ripped off and I don't think it's a scam.
In my experience, the pros are: Often REALLY great savings (particularly if coupled with a special incentive through a site like RetailMeNot, Ebates, etc.), opportunity tor try places I might not know about/otherwise wouldn't try (and some of them have made me converts to full-price dining) and if I didn't redeem the certificate in the time period, I can apply it to something else so don't lose money (most recent example is a certificate I purchased at the Pork Store in SF several years ago, never used, but was able to apply the price I paid a couple weeks ago to some places in Oregon).
The cons, again in my experience, are: Really need to read the fine print to make sure it will work for you (i. E., minimum spend, food only vs. food/alcohol, lunch vs. dinner vs any time, time period in which multiple certificates can be applied...), limited choices of restaurants in your living/touring area and - like many "discounts" - the waitstaff may treat you like second-class citizens when you pull out the certificate (even before you order) so you feel like an idiot.
Overall, though, I continue to buy on occasion.