For the most part I love these group buy sites but am often annoyed with their very buggy sites. E.G. Crowdmass and Ouffer don't have RSS feeds. Spreets coupons don't print out properly. Delays in delivery reported by other users.
CUDO has the potential to be an awesome site. It's backed by Microsoft and PBL, as you can read in Lifehacker. You can't access the site or click anywhere other than signing up to the site. Why do I have to sign up for an account to see a shopping website? Hopefully it's just a bug.
CUDO
Lifehacker Article
Wiki: List of Group Buy Websites
EDIT: Seems it's starting to cause a bit of anger on social networks
Cudo don't actually verify the e-mail address. so you can get away with a fictitious one.