I have three SpaceNavigator 3D mice made by 3dconnexion but I use zero of them. I've tried, but here are the problems I've found -- along with suggestions I've made to 3dconnexion:
1. The round shape makes it hard for me to determine its orientation: which side is "up"?
Solution: Make base mildly rectangular or triangular. (The recent addition of the "arrow" on top of the puck helps, but is too subtle, since my hand covers it up.)
2. I find it difficult to locate and press the thin buttons around the puck.
Solution: Make buttons larger with a more definite click.
3. With six ways that the puck can move, I find I cannot remember which is twist/pull is which.
Solution: Provide stickers for users to place on the 3D mouse or PC.
4. The weight of the portable units is too light, because it lifts off the desk when I pull up on the puck.
Solution: Integrate a thin stiff "mouse pad" that uses the palm of the hand to help keep the puck firmly on the desk.