I'm not a business owner, so I can't talk authoritatively on the subject, but I think there's something in the combination of wide audience and well-known number of pre-paid vouchers that make GroupOn more attractive than old-world sales marketing. Then there's something in their skimming 50% off that's quite less attractive.

I'm not sure this is well-known, and it doesn't appear to be given the comments here, but Groupon offers a self-service option (i.e. not "featured") where their take is just 10%. I feel that is a relevant, and often overlooked, option for those businesses with typically lower margins.

I don't do deals myself, but I find the trend interesting.