I have not messed with one yet but have plans to in the future.
My project ideas for it are:
brewing controller / monitor,
aquaponics controller / monitor,
green house monitoring / automation.
I'm kind of a lunatic when it comes to recording and analyzing data, so being able to have something constantly collect data that I can later graph and analyze is what I would like to use it for. Well that and automation of as many trivial tasks as I can.