They're pretty neat. Note that I use my SpacePilot as a modern alternative to the old SpaceOrb 360, though, which means I use it as more of a gaming peripheral (with the help of GlovePIE and PPJoy).
They go especially well with games like Descent or Independence War that permit full 6DoF movement, as you'd expect. Unfortunately, a few other games start acting up with it plugged in, resulting in crashes and uncontrollability and other nastiness.
However, you definitely should heed that warning about 3Dconnexion devices being rife with planned obsolescence. It actually took some lobbying on the forums to get them to support the original SpacePilot in the 3DxWare 10 beta.