I've been using UnPlugged since I found I was out of power not having noticed I had become unplugged (the MagSafe plug is easy to come out when sitting in the living room and watching TV). UnPlugged has worked great both telling me when I've become unplugged as well as reporting battery charge when I am unplugged.

I also like that it can use Growl. I never really Groked Growl until UnPlugged started using it and now I've found other personal uses for Growl.

I've not installed it on my Wife's MacBook Pro.