I use the TrackIR 4 Pro, and it works awesome. Something i've found, is that using the HAT in conjunction with TrackIR provides great results. It was a kind of annoying looking behind me, so I just flip the hat to the appropriate rear view, and slide my head to the left or right to look behind me.
All the info in this thread is great, and monkey'ing around with the settings took me about a week to get perfect. I found that by editing each Degree of Freedom individually worked best. Disable all but one, set it exactly right, disable it, and move to the next one. I do have the head roll disabled though. It just tilted the screen and was annoying.
One problem I have encountered, is that when I try to close TrackIR software, it hangs, and I have to CTRL-ALT-DEL to bring up the processes list and shut it down manually. Anyone else have this issue?