I have one, although I haven't done a lot of hauling with it yet. Handles almost as good as the bike without the FreeRadical on it. The longer wheel base does affect slow speed handling but I can still track stand it with lighter loads. I have the footsies for it and have taken friends for rides and they were comfortable.
It's really well built and while I wouldn't put it on a lightweight racing frame I don't think it puts that much extra stress on the bike, just the rear wheel when carrying loads.
One thing, it isn't a quick on and off sort of thing. You have to move the rear brake and derailleur to the FreeRadical so you need to either have an extra bike for it or not mind riding the SUB all the time.
-s