While looking for a good sturdy cart, I came across this fine example of a cart with multiple personality! I saw carts for the same price that weren't nearly as nicely built or as versatile.
I've used a Rock N Roller for years. Sturdy as a tank. The large model with pneumatic wheels is great, but a bit weighty. It can give a smooth ride for camera work, too (seat beats not included). If you have a bunch of stuff in cases, you can't beat it.
Sterling Johnson introduced me to one of these a while back, he was toting a lot of cam gear around in Yosemite, and this thing took the trails very nicely. Never considered using it for a dolly though, good idea, Michael.
They're a tough cart, and it's no big deal to clamp extra stuff to them either, due to their multi-capablity