I think it would depend on what info you're looking to find. I find them pretty much useless even for a car or pickup. When you order one up and you don't see anything bad, it just means they don't know about anything bad. People assume that everything is reported to Carfax somehow, when in fact, all reporting is voluntary!
A clear Carfax report means absolutely nothing. There are no guarantees of any kind to fall back on in the case of incomplete information. In short, they're misleading, leading to wishfull thinking. A complete hoax, perpetuated by sales people.
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake