It should stick like glue, are you sure you are attaching it properly? I not might be a defective suction cup, you can get a replacement dirt cheap on eGay.
Got a Streetpilot C510d here, but the Nuvi ones are much better, especially the new ones.
Also, what I have done is hook the charger directly to my car battery (or more specifically, a qualified installer has!) so no wires on show and can put the unit right in the corner of the windscreen out of view. Looks a lot neater.