Had my TrackIR4 for over two years, now have version 5 software.
No problems with either hardware or software.
I tend to prefer the hat-clip to the pro-clip, but that's only because headphones cook my ears after a while.
Also I've found the pro-clip seems to run out of FoV more easily than the hat-clip.
Cliffs of Dover was perfect with simple adjustments to speed and smooth parameters only. Latest game patches seem to make it less perfect than originally however.
Once you're used to it, it's impossible to fly without it, because you keep turning your head and nothing happens!
Totally indispensable piece of kit.