Judging from this thread
is a complete nightmare to implement, despite the new availability of pre-built boards. The insistence that it has to run under Linux doesn't help.
I have huge respect for the technical accomplishments of the Paparazzi team, but it sounds like some all-important hand-holding for regular folks has been lost along the way.
[UPDATE: Speaking of Paparazzi, this
Hong Kong company has fabbed thousands of Paparazzi PCBs (is the market for difficult-to-use open source autopilots really that big???) and has some very good deals on Tiny 2.11s ($185 for the kit including 4Hz GPS)]