"Flush Pro"

I have seen a contraption that mounts in-line in the water intake line, and allows you to install a garden hose to use as a water source so you can run your engine with the boat out of the water. Most recently I saw the device in the Overton's catalog. It's called the "Flush Pro." It says it uses the pressure from the hose to operate a check valve that shuts off the inlet side and take water in from the hose. This would be a lot simple than having to remove the inlet hose everytime I need to work on the boat in the driveway.

Has anybody used one of these things? Are there any hidden catches? I kind of worry that if the thing fails it could cut off water to the impeller, or if it leaks it could sink the boat (like if the boat is in the water over night).

Any insight greatly appreciated!