It depends on how the audio track on the video sounds how well the auto-sync will work. Most of the time it's as good as eyeballing waveforms or matching a clap[board] spike, but in live settings I've seen the camera audio just overwhelmed with the ambient noise and then FCPX might not do it perfectly.
The nice thing is that you can edit your multicam clip before you do the angle view in a project or even after and make adjustments to align an angle that didn't sync up to your satisfaction. I've had to do this a few times. (Editing the mcc itself is a lot easier if you are doing color matching or a slight angle adjustment/rotation on an entire angle's clip.)
There's a lot to like about FCPX. I just wish I had the time to invest in Motion, or the money to spend on a lot of the transitions and titles folks have done!
P.S. The multicam clip feature is good for doing split-screen and pip stuff if it needs audio sync, as well. At least, it's easier for me
