http://www.bluebirdbanter.com/2011/1...south-side-sox
Link says Santos has one of the best fastball-slider combos in the MLB. Only concern is consistency and durability. Not durability in the injury sense, but apparently he faltered more often when his yearly innings count got higher.
Sounds like he has nasty stuff and could be a great closer if Farrell can get the most out of him.