The TrackIR gets a thumbs up from me too. I didn't bother with the Pro Clip (I've heard quite a lot of negative comments about the quality of the pro clip and, despite not being a hat wearer, have grown perfectly accustomed to wearing a baseball cap while playing - despite my signficant other mercilessly taking the mickey out of my "space hat"). Head Tracking adds so much to the game that I can't really put it into words other than to say that it's just brilliant and I can't really play without it now).
The issue (which puzzled me before I bought it) where you're turning your head while still keeping your eyes focused on the screen (which, of course, remains directly in front of you) ... in practice is not an issue at all. Your brain just copes with it effortlessly.
One added bonus which I had not expected is that the "target ahead" function actually targets what you're looking at and not what your ship is pointing at. This means you can look to your right (for example) and target another ship (or planet or USS) while you're flying past it.
I should add that I've also heard great things about the EDTracker which is a hell of a lot cheaper.
Oh, and one other point worth mentioning is that it can be light sensitive. I play in a darkened room but if it's a very sunny day and somebody opens a curtain for example then that can play havoc with it.