They use Facebook connect, so you don't even have to sign up for a separate account if you have Facebook. If you refer friends, you get $10 too! You can also sign up for email alerts or be fans of their Facebook pages so you will never miss a deal. Look what you have missed already if you are in New York!
From a web application developer's perspective, I really like their site. It is clean and it looks good. It's easy to use. It's interactive. It's hooked up to social networks like Facebook and Twitter. I wish I thought of it first :)
If you own a local business, you can do a deal with them! It's great exposure for a day because they put a lot of info about your business on their page while the deal is on. Remember the Filipino restaurant, Isla Pilipina, I blogged about in July? They got a ton of exposure from being part of the iWishLessons.com World Tour, which was a featured deal on Groupon. You can find out more about getting your business on Groupon here.
Some of the really good deals get sold out quick, so check for the deals early!!