Thinking about doing the same myself. I posted on DIYDRONES asking for opinions on switching and got a mixed response, some yes and some no. From what I have gathered the APM is not a plug & play like the NAZA, and you will need more setup, but once you have it dialed in the results are excellent. There is a guy on utube, Peter King, who sells 450/550's fitted with APM quite cheap and puts his parameter files on Google for all to access, so plenty of help available.
From what I have discovered all APM flyaways can be attributed to user error, bad GPS, vibration etc