I think it's pretty great that they've managed to make a Full nelson hold such a devastating maneuver, if you will. It's that they've built it up to be that devastating, not whether it actually is or not. Dusty Rhodes' finisher is a friggin elbow to the forehead, but people respect him for it because he was able to apply it in such a way that made it a good finisher. Same could be said about the masterlock.
Granted it's gotten kind of old as of late but that's because they haven't really done anything big with it since the old days of the challenge. But the fact that it's been able to last all this time is pretty impressive.
I agree that Masters needs a good feud to get him over and to bring him up from mid-carder/filler gimmick to a high mid carder/main-eventer. I think he has the capability to be a main eventer full time, but it's all about getting him over, which he isn't right now.
There was a point where they almost did it, around the time of the Elimination Chamber match he was in, but they kinda forgot about him after and then there was the whole drug scandal.
Anyway, props to WWE and Masters for being able to carry the angle for so long, but he needs something fresh.