Just wanted to share a video I found of the upcoming arducopter auto tune function! No more fiddling around with PIDs to get that perfect loiter. This will be released with Arducopter 3.1 code, but for now you can always try the latest pre release version to test it out.
Now that arducopter 3.1 is released, the autotune feature is available in the stable realease. If you have updated your firmware to 3.1 (and above) and want to try the autotune feature, you can see the official guide here