I can guarantee that there's going to be a teeth whitening deal every week - so why would I ever go back to a dentist and pay full price? Long term, these businesses are damaging their markets and industries. You'll struggle to build a business from fickle, deal-hunting customers.
That said, with things that are unusual I think Groupon is a good solution - a canoe trip, or climbing wall for example. There's less competition and it's a great way for people to try something that they might grow to love. But restaurants - come on!
It's like the reverse of a loyalty card - come again and we'll charge you double.