Co Op is buggy and is pretty much the same as what you get in Napoleon and Empire. Although the Rise of the Samurai Co Op is a bit more advanced in that it allows you to take control of linked clans with common goals.
In theory and practice, Co Op is great fun, until about turn 50 when it falls apart and starts to desync.
There are workarounds, although my current game requires my co cop partner to send me his save every single turn. This would be ok, but saves are around 50 meg, so we use Drop Box and it takes a bit to upload.
The main problem is the lack of support and information. Almost like the Co Op programmer at CA has gone on extended leave since the Empire Co Op Beta and they havn't fixed any of the issues that started way back then.
The workaround of sending your save could easily be implemented into the code. But instead CA release more DLC in form of blood packs, clans/units we already have but can't use and extra campaigns. BTW The blood pack has since been withdrawn in Australia, because when tiny animated samurai bleed, somewhere in the world a fairy dies.
I figured that by disabling steam community in game, Shogun 2 actually runs better. WTF? Why didn't CA tell me that when it was released.
I guess for all my

about CA and the bugs in Shogun, I still can't help but really like the game. It is certainly worth getting to play co op with your friend. Ie Steam Sales.