Leinart has a noodle arm. He won't be a star in the pros. But you never know with all those weapons in Arizona..
Vince Young is obviously the biggest boom-or-bust out of the three. I'm leaning more towards boom. Sure his throwing motion is weird and he played out of the shotgun primarily in college. But the guy just has a feel for the game and he's a natural leader. You can't teach that. He didn't even run the 40 that fast. But his game-speed is faster than his timed-speed..
Despite Leinart and Young being the stars, I think Cutler will end up being the best of the bunch. Cannon for an arm, accurate, good mobility, I can't really find a flaw in his game.