Virtually every professional I know uses one of these devices.
When I first got mine I had trouble with it but tried to give it an honest evaluation over several days. I was about to put it back in the box and sell on eBay when I decided to give it one last try.
I went into the Control Panel and switched the action of the up/down pan and zoom. Within minutes I was flying like a pro. This should be the default setting.
We have 50 of the devices at our school. A good number of the students end up purchasing their own or convince the company they go to work for to purchase the devices.
Students can get for about half price at
http://www.journeyed.com
It is simply not possible for someone not using one of these devices to keep up with me in speed. Especially in operations like routing wiring for a mechanism.
I use with Inventor, SolidWorks, Pro/E (doesn't work all that well with AutoCAD). I also use with Google Earth - takes a bit of work in the Control Panel to get it set up well to work with Google Earth. Whenever I go somewhere new - I already know my way around - all the landmarks and turns as I have allready "flown" the route with my Space Navigator (Actually I own most of the models, but the simple Navigator is the one I use the most as it is relatively portable. I never even bothered to learn the buttons on the more advanced models.