Bully,
I have one and have decided that it is at best a means to keep a good battery good. If a battery is toast, you can leave it on there forever and still have a worthless battery. If the battery is getting a little old, it can bring it back up some. I have a 10yo battery that should be shot, but I put a charger and the little box on there when I can. That is usually for a month or more in the winter. It seems to help.
I have tried it on a battery, that was doing poorly, it gotten pretty bad. After a month it was maybe a little bit less worthless.
Matt