Cech might not be quite as quick and agile as he was in his pomp, but don't underestimate what a massive coup this is (could be!) for us. Nothing to do with it getting under Mourinho's skin or getting one over United - although those things are always nice! - Cech is on another level to any goalkeeper we have at the club currently, have had at the club for a long time or could have signed this summer.
He's a smarter, wiser footballer and goalkeeper now than he will have ever been; his positioning, decision-making, ability to direct and organise his back four are about as good as it gets in the game right now, and those skills will only improve the older and wiser he gets. He'll bring gawd knows how many medals into the dressing room, he'll instantly command respect from every player in it, and that means he can help lead with other senior players, he can demand more when the players in front of him aren't doing their jobs and, I'm sure, given what he's won, he must have a ferocious will to win that will inspire and focus the minds of the younger players at the club. Especially in times of need.
If all of that is not enough, then consider that this is a guy that, despite all the nice words, has been tossed aside by Mourinho for a younger model. It will have hurt him badly and I'm sure he's going to want to ram that deciison back down the special one's throat and show the rest of the League that he's still the best. A player like him with THAT kind of additional motivation is a dangerous animal.