First, what is the company's reputation? You can be sure that if they've done one of these deals before, there are online reviews written about them. Start with Yelp ( and check out our reviews while you are there!) and the Better Business Bureau!
Second, what is included in the cost of the coupon? Is it two rooms of carpet cleaning? What if you want three cleaned? Does it include stain removal? Make sure there aren't any extra charges hidden from you. And make sure they'll honor the voucher even if you do not wish to add on services.
Next, how do their prices compare to the regular prices of other companies? You still need to shop around for prices! In several cases, the regular prices of other companies are still more affordable than the "discounted" ones.
Finally, make sure you know how soon you can get an appointment. You'll be competing with everyone else who has purchased that same coupon for their time.
There are many legit deals out there! But, like any other purchase, you need to do your homework before you make a decision.