Fergie always knew when to let a player go, he must have shown some ability at some point to get there but if Fergie is actively selling someone there's a reason for it. Should never have taken him, Butt or Smith.
Can't think of many good players he let go - Stam's the best example I suppose, and maybe Beckham and Van Nistelrooy. Butt and Smith were so clearly finished.
Forlan did pretty well when he left, but obviously didn't make it in England.