What do you think of portable chassis dynos?

I was wondering what you guys know about portable dynos? Are they just as accurate as a stationary dyno? Or are they off a little? What do they normally charge for one run? I was at a car show (I hate to admitt this to you guys but....it was a Corvette show). TELL ANYONE AND I'LL KILL YOU!! But it was about 6 years ago and the guy was only chargeing $40. Also, if you were at a car show or some kind of event and saw one of these there, would you run your car on one and how much would you be willing to pay? I know at a couple of places that have stationary ones they charge around $75 for one pass.

ANY input is appreciated, THANKS.