I don't know, anymore.
If you just let your PC abruptly shut off, you can have data corrupt and nasty stuff happen. I thought the point of a battery backup was so that you have time to shut off your PC, which is why most of them have a serial or USB interface to shut your PC down normally for you if you aren't there when the power goes out.
I will say though, that sound sounds really crappy when they come on. At least on mine, which is some weird off-brand one.