I don't think in general coupons are a good deal for people. It is mostly clueless people who fall for it. As JoelOnSoftware wrote somewhere, collecting coupons is like working for 7$/hour. With Groupon, even if you save 50% on your Sushi, maybe you spend 2 hours getting to that Sushi place, rather than 5 minutes to the one around the corner.
However, there seems to be an infinite supply of clueless people, so the whole thing might still sail on forever.