A carting woman comes to the Perry, GA UKC shows all the time... it looks pretty cool! The equipment is expensive, though. At least the carts you can ride on are. My mentor got a set for her GSDs, a few of her girls will pull but they're not to keen on hem and haw (I think those are the terms?) just quite yet. They still prefer to go where they please.
If you have a dog with a lot of angles in the hindquarters I'd stick to a four-wheeled cart so there's no downward pressure on the spine and hips. I've only seen larger dogs take on the two-wheeled carts the vendor brings, which look to me to put a fair amount of downward weight on the dog.
A woman with white shepherds uses her dogs under a cart to go grocery shopping and for pulling wheelchairs. Its pretty cool lookin', thats for sure!