Steal or Deal?

Group buying sites are such a hit since last year and is still soaring up until now. Has anyone purchased from one of the several listed here? As far as I can recall, I've signed up for these sites when they were just starting. Bought a voucher for a minimal amount only to see how it goes and if it is for real. I guess it's for real coz a colleague of mine purchased her hotel voucher from one of those sites when she took a vacation in Indonesia. She said it's quite a steal. Lucky! I found one yesterday where they were introducing a yoga class for a minimal amount. I want to try yoga. I can even find cheap accessories online from yoga mats, thera bands, etc.  I wish to be motivated to start doing yoga. I also wish that if in any case I buy a voucher online, I wouldn't forget that I did so. Haha! I've wasted a couple already. Tsk tsk....