I am thinking of buying a SpaceNavigator but on the website it reads that the SpaceNavigator should be used in one hand and the other hand for the mouse.

How would this work on blender? as most of tools are done via the keyboard

There is a Google Summer of Code project this year that is for improving tablets and the SpaceNavigator in Blender.

I had a SpaceNavigator that I eventually dumped a bowl of soup on (bad hacker, no cool toys!). It wasn’t really all that useful in Blender (2.49) – I ended up mostly using the numeric keypad to rotate and pan anyway. If the new SoC project makes it more useful, I suspect the way to use it will be as a “reach over with your non-mouse hand, pan/zoom, return to keyboard” kind of thing. The reaching distance isn’t too much further than reaching across to the numeric keypad, so I could see it working. I’ve almost bought a replacement SpaceNavigator several times but I don’t really think it’s worth the money for me.