I know most of you guys are running rigs too big to think about even trying one of these...
That being said, my 24' Pantera would be a good candidate for one. Have a very tight alleyway, and the 12' gate only opens 10'.
It was tricky (to say the least) getting it into the backyard here at the doublewide.
I've seen plans for these for sale on E-bay, and to buy one from one of the Manufacturers is about 1k.
I'm not looking to buy one tomorrow, but thought I'd get a little info on them if this is something that I should put on my "wish" list.

If you're not sure what I'm talking about, they are a powered dolley that helps moving the boat/trailer around detached from the tow rig.