Seconded. Track IR 5 was brilliant when it came out (and Track IR was really the only option), but having to wear a hat and the funny reflector attachment was slightly uncomfortable for long gaming sessions. Worth it when there was no alternative. (BTW the main transmitter/receiver unit you put on top of your monitor gets HOT!!!).
For a short while I thought FOIP for head tracking was brilliant - I had a webcam on my gaming rig anyway because I use the same monitor for my work computer. Just have to switch the USB lead from one computer to the other. It works surprisingly well, if you already have one. Not sure I'd spend the money on this if you don't have one already, unless you can justify buying a webcam for other reasons.
Tobii, however, works MUCH better than either of those do. It's just awesome. I'd 100% vote
Tobii if you can afford one. There are sometimes sales with ~15% off, though I don't see one at the moment.