I work with a space navigator for 4 jeers now and I can't imagine doing it any other way. My first experience with it was bad only to discover that it was my failure to set it up properly. Symptoms of bad setup are for instance movements with the device translate counterintuitive to scene movements, ore movement in some directions are too sensitive etc.
The device has many settings and you should take the time to make yourself familiar with them very well, if you manage to setup the device properly then you will fly around your scene like a bee (with great ease) in no time.
I highly recommend working with it as it makes your mouse completely free and independent from navigating your scene.
The thing works very well in 3ds max, don't know how it performs in other packages. Work's also relatively good in 3d coat and it is also nice to use with Google Earth. I've experienced some problems with it in Mudbox, integration seems to be flawed in there. Sadly there is no support for it in Zbrush.