I use TomTom Navigator on my Pocket PC, in a mount. It is easy to navigate around the basic navigation UI, because all the buttons are bigger than a finger tip. The trick is to rest a couple of fingers on the side of the mount and then press the button with another finger.

If it wasn't in a mount it would be extremely dangerous to use.

Looking at the Musicvaultz's UI everything looks far too small to be useful. They need to dump the standard Palm UI widgets and think a little more about the control system.