I don't use those things, they're icky! There's always that piece that hangs into the toilet, and it acts as a wick to draw water upward.
Plus I was watching a show (probably Oprah) where an expert was saying that they give a false sense of safety, they really don't protect from any germs or anything on the toilet. Besides, it wastes trees!
A girlfriend of mine uses them religiously, but she

when I told her about the Oprah show. I remember her telling me that when she goes to a bathroom that doesn't have them, she "hovers". And with toddlers, she actually holds the child over the potty rather than have any part of them touch the toilet. OMG, my arms are just not that strong!