For example, now I am looking for a vacation package for the summer, Groupon emails "all inclusive" packages in Dominion republic, mexico, etc. The price is similar to Costco's packages, but there are more choices.
For restaurants, we have bought Vietnam cruisine, Greyvey, middle eastern, India food coupons from Groupon which we normally would not go. Since groupon has the deal, we have tried many non-traditional dinning outs.
The quality - well, it does not put very high quality options on sale. But good enough if you want to try something different
Oh, last week it sells 4-5 star hotels in Niagara Fall and we bought that also... We would not have thought about going there again since we had been there twice. But with $69 per night, we don't mind spending two days there in the Canadian side
It seems I am the frequent user :) Last summer I used it to buy a one-week fencing summer camp. The normal price for the camp was over $300. Groupon had it for $190 or something.