Good part: discounts are usually very good, at least 50% off. There's a new deal every day, but they don't always expire in 24 hours, so you can come back to it if you need to think about it first.
Bad part: if you live in a large metro area, the discounts offered can sometimes be for areas farther than you're willing to drive or in places you're unlikely to frequent. (I get weird offers all the time for belly dancing classes or tanning salons, which are just not something I'd do, and half the time the restaurant deals are either small, locally owned independent places that I've never heard of, or are too far away for me to drive to if I'm not 100% sure I'd love it. And they're usually non-traditional too, like international cuisine or sushi restaurants - I hate fish!) Also, it's not as easy as just printing off a coupon. You have to BUY the coupon, on the spot. So, if I get a deal for 75% off a haircut at my local salon, but won't need to get my haircut for another two months, I still have to purchase the deal upfront. And, if you get annoyed by SPAM, then getting the daily deal can sometimes be annoying, since they do send you an email every day.