See through clouds?
The flir only senses temperature differences. If you are in a cloud you only see a blur on the screen. Like gulfstreamaviator said it is a great complement to the radar as the cloud is a different temeperature than the air around it. Great for avoiding buildups, especially at night.
I am still wondering if anyone has hand flown a sidestick through moderate or severe turbulence. There is someone over on the main AF447 thread bringing that up now, but I still haven't heard from anyone with any experience with this.