I have dozens of battery tender Jr.'s on all my machines on the farm. They work great. The battery tender, regular model, is even better, because it is temperature compensated, meaning, it keeps a higher voltage in cold temperatures. It also has about double the amperage of the Jr. model.
However, as you said, if you've got your batteries charged already, it will keep up a few hundred amp hours of 12v no problem. In fact, it will charge them eventually if they are down, but this will heat up the transformer somewhat, and I always make sure they're well ventilated if the battery I hook up to has a good ways to go to be charged. I even keep the golf cart batteries topped up, without fear of boiling over, by using 2 24v battery tenders, mounted on the cart.
I've never had one wreck a battery, but have had a couple fail when I put them on some motorcycle batteries that had a shorted cell....I just thought at the time that they were at low state of charge. I know, I should always check the specific gravity, but stuff happens and you get distracted from all the things you should do.
BTW, I always keep a Battery Tender Jr. hooked up to the RV chassis battery. It is plugged into the coach 120v outlet and recovers the battery in short order if I, say, run the jacks without starting the engine while parked and plugged in.
