I've had TrackIR for a few days now, and WOW. My WW1 gunnery stinks(more incentive to get close), but the payoff in immersion is well worth it. In both WW1 and WW2, my SA has skyrocketed, and now I do not fear rolling scissors nearly as much.
I got the trackclip pro as well, and ended up offsetting my TIR to the left quite a bit. Seemed to track better that way.
There's a few features I wish it had. I wish pause were equal to freeze. Either a toggle or momentary switch would be fine. I also wish a key combo command were available to swap between two profiles. In the case of AH, WW2 and WW1 profiles.
If you're thinking about getting it, get it. If you're worried about cost, freetrack is a viable option, so I hear. You're accuracy and SA will suffer for a bit - about two days for me. Then it's like someone switches a light on and you'll never want to go back.