WE use them on a few tanks.  Problems are the arm that the float attaches to is difficult to keep from slipping over time.  You also need a good mount either in the tank or a post next to it completely fenced off.  You also need the angle set on the float so that it doesn't bump the pump on and off constantly with a little water movement in the tank.  We only use them on wells that either are too poor to be run 24/7 or cost too much to run 24/7, otherwise it is far better to just run tank continuously.   Also remember that you are going to need 120V wire running all the way back to the well with that setup so well and tank need to be relatively close.