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There are a few practical issues in terms of everyday use. First, the passive clip had a tendency to fall off my own hat (though this might be the design of my hat), while the alternative TrackClip Pro is rather cumbersome and weird looking. Get it out at the average LanParty and it might look like you've tried unsuccessfully to spawn antlers. Also, I had to either sit lower than my usual sitting position or raise the monitor in order to get the camera and clip to work correctly. In my comfy gas-lift office chair, that's not a problem, but there's a chance it might be for you.
A utility is provided for setup and calibration, and you can use this to get a feel for how the system works. There's a sphere view to give you an idea of how the tracking works in relation to your head movement, while 3D wireframe models of a human head help you check how your movements will be reflected in-game. There are easy adjustments for smoothness and speed of movement, and you can create custom profiles and set them for use with specific TrackIR supporting titles.
Here's where there's some room for disappointment: while there is a facility to make TrackIR function as a no-frills mouse-look, the technology really only works with games that have been designed or patched with the system in mind. Looking at the list you'll see a number of big brand names - ARMA and ARMA II, Flight Simulator X, GTR2, Race 07, Tom Clancy's HAWX, Colin McRae: DIRT, Race Driver: GRID and others - but the list is mostly restricted to serious sims. TrackIR would lend itself to other games - Far Cry 2, Battlefield 2 and Ghost Recon: Advanced Warfighter spring to mind - but they won't work correctly unless NaturalPoint can convince the developers to do the work required. The company is clearly working hard to do so, but while budgets stay tight and the technology remains niche that might be an uphill struggle.
And that's a shame, because where it works TrackIR can be very impressive indeed. I tried it with a selection of games, including X-Plane (the popular hardcore indie flight-sim) Colin McRae: DIRT, Arma II, Race 07 and Future Pinball. While the technology takes a little getting used to, it definitely adds a new layer of immersion to the experience.