I personally only ever used it for a discount at businesses (restaurants) I already went to. It was lose-lose for them on my purchases, but at least I still went afterwards.
Many Groupon customers would just use the coupon, and never return. This is a total loss for the business, because the coupon was often below cost for the restaurant. Note that the only things that really remained forever is the things that had nearly no incremental cost for the business to give out (free trial for SaaS-style things).
1) Deals are not good. That is, the price reduction is not real or comes with reduced service.
2) Problems with honouring the purchase. The experience is not as purchasing something at reduced price from the shop, you need to go through separate process and the vendors act as if you forced them to sell you the deal. Some try to upsell a bit too hard, degrading the experience if you refuse. Some claim that your coupon is not valid and you need to deal with customer service to resolve issues.
3) Social stigma. It's uncool to take someone somewhere with a coupon, so all the deals for two are not good.
IMHO it could have been like a trail session, have it been framed correctly. Instead, it was structured as race to the bottom from every side.
The stigma of using a coupon didn't bother me so much, most restaurant servers were always cool about redeeming them (and of course I always tipped on the pre-Groupon bill.) But I would sometimes get an (unintentional) negative vibe from higher ups, like great another money-losing customer that I'll never see again. That reaction is what discouraged me from using Groupon, if it just ends up generally being a bad proposition for businesses I'd rather just avoid the whole thing.
Oh man, this reminds me I still have a much of credit at Restaurant.com, which I stopped using a few years ago too...
But, yeah, online if there's a new customer discount or a periodic 20% off one item discount I'll use it to buy something I would probably have bought anyway. For most in-person things, I don't go out of the way to find/use coupons unless it's a very transactional thing like a museum admission.
For example, most of the restaurant coupons didn't apply to the real menu, and was usually a "special menu" that consisted of whatever cheap crap they could toss together for pennies (always pasta) and the worst bottle of wine they could source. After getting burned a couple times on that we never went back.
The equivalent of a custom-made Black Friday TV model so Best Buy could claim "60 inch LCD TV for $199".
Groupon sold itself as OS for local commerce. I see that as a 17 billion idea.