Just got my TrackIR yesterday ... OMG, it's a game changer!
To any headtracker fence sitters out there (and I've been toying with the idea of getting mine for months) - get one, they add more immersion to the game than I could have possibly imagined.
Couple of quick tips from what I've learned so far.
First, use the training scenarios for your first few goes. I nearly stacked my Asp the first time I tried to land. It takes a little getting used to. Not much mind you, it's surprisingly intuitive (especially the thing of keeping your eyes focused on the screen even tho' you're turning your head, I barely noticed I was doing that).
Next, take note of the key that's mapped (within the TrackIR interface, not within the ED keybindings) to Pause (a toggle that turns head tracking off). It's really useful to be able to hit this during things like docking or when using the station menus. Actually, better than a key, what I've now done is used the joystick button that I previously had as my headtrack toggle.
Also, despite really liking the idea of it tracking the roll, X, Y and Z axes, in practice (as a lot of people have already said) it's probably better to uncheck these options.
Lastly, and I've only just discovered this so not much time to play with it yet, in the TrackIR interface where you have the different profiles like one-one/smooth/fast/slow/etc, you need to hit the little apply button to apply that profile to the selected axis. The one thing I'd been struggling with was how "twitchy" it was when generally just trying to look forwards. By applying something like the Smooth profile, I'm hoping it will dampen the motion in the central deadzone area.