I've used them for years. I'll skip the obvious stuff and give the bits that may not be obvious.
The fact that their coupons never expire is great. That means that you have to keep track of them and exchange them, but if you persevere, once you have a pool of coupons you can stop buying any and just move around your credits.
Always check that the restaurant will accept the coupon in advance. I can't tell you how many times I have found a neat restaurant, bought and printed a coupon, and headed there during its stated business hours to find a vacant building (the restaurant failed a year ago), or been told by a sneering host that "they don't take those things any more". I'm not a restaurant, but I've heard they pressure the restaurants pretty hard, and it is a better deal for the consumer than for the restaurant past the early days where the restaurant is new and needs to promote.
Their web site is terrible. In theory it is well-thought out, but is often too slow to use, or just buggy. If it works, great, it can do almost anything to help you organize your coupons or find restaurants. Open one window there, another with Yelp, and go research your area for bargains.
Watch the fine print. Holidays, no way. If it says you have to spend $50 to use a $25 coupon, does that include what you spent on your drinks? Probably not. Watch if it only for dinner, or doesn't apply for specials. Read the restrictions very carefully, And watch your bill, in some cases they add the tip in, and if you aren't careful you can tip twice.
But overall, if you are careful and methodical, it's great.