I've used my 95 GT wagon for dragging three or four of us and a 14'6 sea nymph with an 85 yammy on the back around for a few years now. Towing a decent weight definitely highlights any issues your car has (e.g stuffed coilpacks) and after three years of towing the back of my wagon creaks and groans even without anything on the towball - it's definitely taking it's toll on the shell. In fairness, I've abused it as a tow car but mechanically it's all been good....
To give you an idea of what you can use them for - I've used it to tow all the following in one go (on the beach before i get flamed again for doing illegal things on a public road ;D) - my sea nymph which is a heavy old lump, with an 8ft fibreglass hulled inflatable and 8hp mariner shoved in the boat, all with four fat adults and a s*** load of gear in the car and it's been fine. But then, you need to drive within your limits and be ridiculously aware of what's going on in front of you, cos hitting the brakes hard with all that on the back definitely makes your arse go on suck..... even on sand lol !!