I haven't used one in relation to bathing horses, but have used one to make hot water for washing dishes/bathing at a hunting camp.
They can be a bit tricky to get working right between the water pressure and the heat element -- if you have your water passing by the element too quickly, you won't get hot water. The cart doesn't seem to come included -- if portable is key for you, it probably would be worth it to order the cart at the same time.
I have one. Works great. It was really nice to have around last spring when we forgot to get our house propane tank filled. I would have had to take cold showers for 3 days. Instead I had my handy dandy water heater to use.
I have one designed for camping in my barn...I use it for all my hot water needs. I think you can get a better price than that, and if you want it portable, mount it on a dolly!
But, the propane powered, on demand hot water is excellent! One thing to watch out for is what it does in freezing conditions...some are not designed to work if the temp is below freezing...fine for camping (I guess) not so cool in the barn, when I need hot water!
I really think I might try the Insta Hot (or similar)... seems like it might work out perfectly for my set up...
I got this one from Camping World, and made my own little rack for it with a utility cart and bungees. I tried using it once so far, and I can say that the hose it comes with is WAY to short. This means the scary hot water heater thing, which is not silent, is too close to the horse and the water is too hot.
We got a longer hose that DH needs to install, to see if that will help both with distance and in cooling the water down.