I've used quite a few, and I would say it depends entirely on who the experience is with.
For example, I bought a voucher for a haircut, and I felt that I wasn't being treated like a proper customer. The cut/blow dry was rushed, and the "conditioning massage" which was bigged up on the voucher, lasted about 30 secs. I felt I paid and got a £20 experience, rather than paid and got the promised £60 experience. However, when I complained to Groupon, they gave me £20 of credit to use with them.
Another thing to look out for is who is providing the experience - do your own research. I know of someone who was approached by Groupon to offer an experience, and the sales pitch was very misleading, implying she'd had years of experience and that her 'normal' price was 80% more than the offer, when in reality she'd only just qualified and had never charged that amount.
If you do go ahead, I would suggest booking asap, as some places get a bit fed up with the groupon customers towards the end, as they know they are not making much profit.
Do your research; call the restaurant, find out how much it normally costs, would they serve out the same portions etc or have they got a special 'goupon' menu which normally means you're not getting such a great deal.
I think there are some really amazing deals btw but these are things you need to watch out for!