That is not slightly disturbing at all.
518s come with teflon feet right? I don't have a desk big enough for a suitable mouse pad so I go without. But I am saving up for a bunch of different pieces of furniture and a real desk is on that there list.
My desk has an incredible amount of varnish on it. If you keep it clean it is quite the mouse pad. But it doesn't compare to glass

.
unfortunately, the feet on 518s are plastic, or at least plasticky. on the g5 you get teflon.
as for space, it shouldn't be an issue. a 20 dollar Steelpad S&S improved (which is still one of the best pads around) is only about 1/16 of an inch thick, so it can easily go under your keyboard.
oh, and glass pads are terrible. i had one and the only thing i use it for is rolling spliffs and doing coke