Just moved this into the DIY thread. I've been building this for about a month (finally got around to putting it together yesterday and have it at my desk). I made it from an e90 M3 seat that I bought as a pair and pulled out the rails/ motors/ wiring and mounted it to the base of an office chair. I'm going to update the thread with a few more photos of how I mounted it etc.
I made one previously with an e46 seat and have the build thread for that here: http://m3forum.net/m3forum/showthread.php?t=428840
Let me know what you guys think. Yes, it is surprisingly comfortable!