I have one of those horrible battery operated metronome/guitar tuner thingys, and while it can be useful, I am currently waging a war on metronomes (not their fault, I just can't play properly with one, it annoys the hell out of me.)
I would prefer a normal to a battery operated one, but it is very rarely that I use it, because I like rubato. I just can't do that in what I'm learning at the moment.
If that last bit doesn't make sense, it's because its 1am here. Argh, I've gotta get up at 6 later this morning.