I used lock on a few times but it's terrible. Once you press the button, you have to wait a full second or more before it kicks in and targets the creature. I find just using the virtual d pad to swing the camera around is quicker. If they made it instant instead of a delay it would be awesome.
"Can you draw two M&Ms fighting with katanas?" -Me http://img16.imageshack.us/img16/9606/mmfight.png -aDubiousNotion
I've found it the opposite. It seems to be as quick as the usual view change (original L button use), and I'm definitely finding it easier than having to constantly shift grip to get to one of the D-pads. Still, I suspect different people will find one way easier than the other.