1st let me say I've never used one.
I have heard very little good about them, and what I have heard good about them is that the concept is good/great, but the execution usually leaves you wishing it had been yours(the execution, that is).
In other words don't buy it until you've seen someone elses in action, preferably atleast twice, and including riding in the tow vehicle on the Freeway.
I've heard they do everything from rattle and chip paint/Galvanizing off(believable) to cause on freeway porpoising(maybe, but more likely due to misloading) to coming off and causing an accident(owner stupidity).
Rather than get one of these for his Sailboat, 15 or so years ago, my Dad got a fixed extension welded into place and because it was
re-engineered right to start with it was very good. ( if this isn't re-engineered right it can cause big time Porpoising and fishtailing.)
I hope this helps.

Tight lines,
Wade