I had them on my donor car, (two tons)for a while when I had a shop. Great to be able to move in any direction they want to go. Now I have one of them on the front and one on the rear of my kit car since it doesn't have any running gear in it yet. does the job for me.
I did break off one of the casters, when it did a large drop off a trailer, because my helper was impatient and wouldn't wait for a ramp. I had to re-crimp the caster together after I found all the ball bearings. They have been on sale lately for about $49 each at HF. I have also seen imitation type Go-Jacks at HF for about $60 each. They are a bit more flexible, if they work as well as the real Go-Jacks, since you don't need to use a jack to lift each wheel to insert the dollies. You can also remove them and use them on another car if you need to, without moving the car to fit a jack underneath. No matter how many cars you have you only need one set. I think they use friction washers instead of a ratchet like the Go Jacks.