After much discussion with the techs at NaturalPoint, I decided to plop down $148 for the TrackIR 5 hardware. I am also running TIR5 Beta 4 software as well.
I must tell you honestly that this is a HUGE upgrade over TIR4. Much more than I thought. The primary difference between TIR5 and TIR4 is "increased resolution." I really scoffed at that and really thought it was nothing but marketing hype. I was so wrong. Turns out, the great work Mace has done building a TIR4 profile that makes TIR4 so much more responsive than it is with the default profile essentially was an effort to overcome a lack of resolution. He may not even have realized that. I certainly didn't. With TIR5, because there is greater resolution, there is much more responsiveness, but it is also much much smoother as a result too. I was really amazed right away. In fact, I loaded Mace's profile (TIR5 now allows TIR4 profile import) and it was unusable. The response is SO fast that it is impossible to control. That's because the resolution of TIR5 is so much higher that the view just jumps all over the place! The default TIR5 profile is every bit as responsive as the one Mace did for TIR4 because the hardware is just that much more precise. You'd have to experience what I'm saying to really understand. Within fifteen minutes, I was able to get the profile adjusted for AH in such a way as I have never experienced. I even left the "Roll" axis enabled which allows me to look around the tail better. Many of us have left that axis disabled because it caused so much confusion. With TIR5 however, the movement is so easy to control and natural feeling that the roll axis is just as natural as the other axes.
Personally, I'm loving this thing and am glad I got it.