I had the Arkon windshield mount for my Pearl, and it worked great -- it did not move at all.
When I got my 8820, I bought a Brackerton mobile grip-it universal mount from Bestbuy (would fit the 8830 as well). The cradle works great, but I hated how the thing that held it to the windshield vibrated so much that I couldn't really see the maps on the blackberry when I was moving more than 30 mph or on a bumpy road.
The Brackerton cradle had two plastic tabs that held the cradle to the mount, the Arkon only has a single tab -- BUT, the tabs are basically the same. So I shaved the tab off the Brackerton cradle and it works flawlessly with the Arkon mount. Maybe a bit of a kludge, but it holds the device perfectly, and it doesn't move at all.
Arkon: Arkon Car Mount for BlackBerry Pearl 8100 | Sprint Sanyo M1 Mount | BlackBerry 8100 Car Mount
Bracketon: The PH-Grip-iT for Portable Mobile Electronics