Does anyone have any experience with those bike sweat guards I see in bike catalogs? I was wondering if those things are worth buying for my time on the trainer.

looks cool, the only problem is that it restricts the amount of areas that you can put your hands, so i think that is the only compramise.

just put a towel over the handle bars..

I've got one. I use it and a towel. The sweat guard protects the top tube and headset area pretty well. It doesn't sag like a towel so you don't have to worry about dropping it or having it get around your pedals. If you're going to train inside a lot I would recommend it.