I just purchased a JBJ ATO on Friday. I seems to be working well and is very easy to setup. My only advice is to make sure that your topoff water supply will not exceed your total system water capacity. The float system seems to work well, however, the fear is that their might be a system failure and increased chances of overflow.
Also make sure that your top off source is covered and much as possible. Evaporation is going to be an issue if you don't.
Last but not least, Mode A on the ATO seems to be the most realiable why to set the system up. The other 2 methods appear to be 2 more ways that they thought up as to how to use the system to make it appear more versatile.