I have the Holux GM-210 stuck on the roof of my car for around a month, and it works like a charm. Its been exposed to extreme weather (storms and also under the Aussie sunlight!) and it is still working well. I personally think Holux did well in the waterproof thing (I could be wrong as I havent really opened it up and dont know whether water managed to sneak into the mouse. Hope not!)
See it in action on my roof here:
http://hihidamon.gfoto.com
Warning!! I accept no responsibilities or whatsoever for damages to your gps and carpc if you put it in the rain. Do this at your own risks!