Pros and cons
Cons (because they're smaller): He's old and no one knows what shape he could be in. (my guess is: HE'S NOT THE FREAKIN FAMILY GUY THAT'S FOR SURE)
Pros: Knows the system (oh this is such a cliche in Boston but to hell with it, we're the only team that has a system built for veterans only), is tall, spreads the floor, did somewhat good on Center position and added A LOT to our defence 2 years ago as he knows all the tricks that we could witness, woke up in the playoffs just like the rest of the team, no veteran cares about regular season, they all play basketball for the playoffs, and most of all - he's an enigma, nobody knows what he can or can't do (with KG, RA, RR and PP, you just can't be bad).
The zombie mode Sheed from regular season turned into inspiring mode when playoffs started. He initiated quite a few runs and hit quite a few daggers against Cleveland, Miami and Lakers.
If this all doesn't persuade you, then this might: He's the only big-name player to come to Boston willingly since the summer of 2007.