I'll be honest: it's a lot of fun, but if I lived 100 lifetimes, it would never save me time on balance. ;)
I also use one to run stratux as another poster mentions. That one saved ~$650 vs buying the COTS solution.
http://bemasher.net/rtlamr/signal.html has information on how it works, but you don't need to do/know anything about the protocol to get it to work.