I use them almost exclusively for hotels. End up staying a some really nice hotels, really cheaply. Used them once for airline with good results. Pretty much have to be willing to lose 2 days of your vacation if you use them for air, as you could get a late flight going down and an early flight coming back.
I wouldn't use them for car rental as you can get just as good of deals conventionally and you know that the company you are renting from will be at the airport (no shuttle).