There's a local Indian eatery my wife and I enjoy from time to time. The service was always excellent until the time when our visit happened to coincide with a Groupon special. We walked in and noticed right away that the place as far busier than usual. Unfortunately that had a detrimental effect on the service, which stank and we were pissed off once we learned it was a Groupon night, as we paid full price for crap service and we are regulars.
Too bad for the restaurants in large cities which get sucked into trying Groupon. You place gets filled for one night with cheapskates, who then move on to the next Groupon deal restaurant the following night, and so on... You f'd over regular customers for a bunch of 20-somthing cheapskates - we have not returned and may not.
The whole point of coupons and specials is to get people to try your restaurant and then come back if the person likes it. However with Groupon I doubt very much this works as the only people using Groupon are the types that are eating at the next local Groupon special as all they care about is the deal.