Chelsea will get 4 good years out of Deco who is one of the best playmakers about, he doesn't rely on pace so he has nothing to lose apart from wear and tear as he ages.
I was reading all the articles coming through there an a lot say he was too old but wether your signing Deco at 31 and getting 4 years out him it wouldn't matter if he was 25 as most players only stay at teh same club for about 5 years nowadays anyway.
Deco is as I said one of the best technically gifted footballers and playmakers in the business - who is no worse and arguably better than Arsenal's Cesc Fabregas and Chelsea have made a great signing even though im not the biggest Chelsea fan by any means Deco is a great signing for you. There are only to my knowledge 5 players on his level or better to link midfield to attack in the game Xavi, Pirlo, Kaka and Fabregas.
He'll give you a balance in the midfield you can now play Essien, Ballack and Deco in the middle which is better than Essien, Ballack and Lampard because Lampard and Balalck are too alike and no-one offers Deco's vision and passing ability to sneak passes through to the strikers.