I built the head trackIR unit the was free, tried face trackIR a bit, in the end I bought a real TrackIR. While I did get the "free" versions working well they did get a hick-up now and then plus depending on when I had them up and running sometimes my computer really wasn't strong enough to add in that function.
Over all, free versions work well, but save your money and buy a TrackIR. It works much better and is worth the price if you like using head tracking. Some people don't which is why I tried the free versions so much..... just to be sure before I spent the money.