The WSJ has an article about "Groupon's Boston Problem: Copycats" (here, paywalled). A better title for this article should be: "Groupon's Boston Problem: Dealseekers".
As I discussed before, Groupon's merchants benefits only if the coupons are targeted to those customers who are new to the business or existing customers who would increase their total spending. Dealseekers are the bane of (most) Groupon merchants but also the fuel for Groupon's growth. As these merchants point out, dealseekers show up and spend the minimum amount to qualify for the Groupon deals. If the coupon offers 50% off if you spend more than $50, then the dealseeker spends exactly $50.
This is a less obvious example of adverse selection in marketing. Credit card companies have long known about this problem - the smart ones don't compete on rates. Whoever offers the lowest rates will attract the worst-quality customers because other banks are not giving cards to them. Same thing with insurers: that's why Warren Buffet always says price to make a profit, not for market share.
In the case of competing offer-deal sites, i.e., the copycats, the winning site is the one that loses the merchant the most money.
In the same way that "Restaurant Weeks" are here to stay, Groupon will serve some merchants and some customers well. But it's important not to mistaken those for the average merchant or the average customer.