Hi Graham,
I have experience of a very similar product called DelanClip - its not the hat you mention but a clip which attaches to the side of a set of headphones. It too runs using a freeware tracking application and works brilliantly. for the princely sum of £35 I was up and running with 6dof headtracking and have used it extensively in both driving and flight simulators - from all the videos of TrackIR i've seen in action I can't see anything which makes me think its much more technically advanced (although I may be wrong - I often am!)
I also use the old PS3 eye, which is the camera being sold on the Trackhat site, they are cheap and run at between 60 and 120fps depending on resolution so plenty quick enough to track head movement.
Andy