I've posted elsewhere about this but I'll summarize here...
The TrackIR is the best thing for simming since color monitors were invented, and the best thing for sim racing since the first wheels were offered.
IMHO, if everyone online used the TrackIR then we would cut the number of total track incidents in-half. The TrackIR's ability to raise your situational awareness, to keep eyes on the cars surrounding you, and to peer through the turns to pick your line and apexes is simply the most involving sensation offered in home simming.
The details:
It takes about a week to get used to. If you have Vertigo or are super sensitive to motion sickness then I do not recommend it. The resolution for every axis is extremely tunable, and the included software is easy to use.
I highly recommend getting the TrackIR 3 with the 6 DOF expansion. Not all the racing sims support full axis yet, but they will. And if you flight sim then you'll really enjoy the 6DOF (imagine being able to peek around your nose and prop in a tail-dragger, or peek out and down over your cockpit walls to see the ground below). Awesome stuff.
Check the bigger flight sim retail websites. The often run specials and combos for the Track IR.
Cheers
-Smoky