[msimon785]

I own one. I find that when in the process of CADing, they are more of an annoyance than an aid. Without extensive practice, I find it very difficult to achieve a level of precision.
However, if I want to show my assemblies off in a presentation style, there is no smoother way to do so.
They require a huge learning curve. You will need to play around with the settings until you figure out what is most intuitive for you. If you are willing to undergo the curve, then they are probably worth the purchase.
Also, the SpaceNavigator only costs $79.99 for those affiliated with an educational institution.