I have had three track clips, first one was broken when I took it out of the packaging, the second one broke after a month, the third one after a week (or something like that), I then stopped even bothering returning them since I found almost no practical difference in tracking from the cap and the track clip, I thought maybe I could use it and have some background light but the track IR still won't let me filter it out as the camera just wasn't good enough.
Also the track clip is for some inexplicable reason (to save money) powered by a USB cable, which is to short and always in the way.
On top of that they charge 50$ for it, which is insane considering.
The cap works just as well, is included, does not come in the way, and does not break. On top of that I don't have to squabble in the store every other month to get a new led lamp to put on my head.
The track clip pro is a cash grab, it's so cheap to make that the money they make from it far outweighs any badwill they get, they afford it because naturalpoint squashes any kind of competition and because of that there not being much of a choice when it comes to head-tracking in games.
That's pretty much my take on it.