They usually come with a very highly rated knuckleball, like 18-20 on a 1-20 scale, AND they usually have high stamina.
What I like about them is that a pitcher with one pitch rated that highly CAN break out and pitch an absolute beauty here and there.
Of course, if you are lucky... you get a game-generated knuckleballer like this:
The guy has 355 career starts, 169-111 3.87ERA, 37.5 career WAR, 61 complete games and 17 shutouts... Yes, his last years have been a bit tough, but for some reason he has never made more than $11.1M per year in a standard financial enviroment...
PS: OK, RchW beat mine big time XD