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.
However, much of Groupon's appeal is the incentive to try new restaurants or do new things. The sushi place around the corner may be good, but you're probably not going to remember your nth visit.
You've got a point. In fact with the right context aware notifications, maybe Coupons are OK - like if you wanted to buy X anyway and you automatically save money.
Then again if it worked that way, there would be no incentive for businesses to offer coupons...
However, there seems to be an infinite supply of clueless people, so the whole thing might still sail on forever.