Have you ever turned down $6 billion? Groupon did when Google offered to buy the company.
Groupon will print you for a coupon for a local business - if enough people online show interest as well.
Groupon incorporates some of the key lessons of internet business: long tail marketing, greater efficiency in uniting sellers and buyers and social networking. I want you to think about that if you're starting an Internet company.
Here's a nice Analysis of Groupon in The New Yorker.
The article criticizes Groupon a bit for not being truly unique (there are already competitor sites) and for that greatest of sins in today's world: employing thousands of ad salespeople (hey, there's a job for you after graduation).
However, they do praise Groupon a bit for clever organization, adcopy and name recognition.