I know someone who paid for two Groupons, one for a wine tasting and one for a Peruvian dinner. He has called multiple times over several weeks to make the reservation and the restaurant will not answer the phone. He's talked to Groupon about it, but so far nothing has been done. He's not giving up but is understandably frustrated at this point. His next step is to go down to the restaurant in person (assuming it is still open) and ask to speak to someone.
As for the wine tasting, the vinoteca called him two hours before the scheduled tasting to say that the sommelier couldn't come in that day. He asked for a more detailed explanation and got brushed off. The next available wine tasting was more than two weeks away.
So just know that you are paying in advance for something you might not ever get (or might have to jump through hoops to get), not necessarily at the fault of Groupon, but rather the businesses that arrange deals through the web site. Strange, since the businesses don't get their money from Groupon until the coupons are validated. As always in Argentina, your mileage may vary.