Rui Carmo
The short story is that I needed something that I could use with my left hand to play Urban Terror without bashing my iMac's keyboard in and that allowed me to keep my keyboard-oriented reflexes (so that I can play without it if needed be). I had seen these around, but it wasn't until I saw there were Mac OS X drivers available and figured out how the configuration worked that I decided to pick one up (the fact that it was being discounted helped too).
After around 10 minutes testing, I forgot I was using it (that is, until I tried to drop a weapon - I forgot that binding). It's that good.