Pros: Compared to other Total war games; a better AI, more attention to detail/historical accuracy, while still the sort of engaging Total War game we love.
Cons: Realm Divide is an annoying pain in the bum
Shogun II manages to avoid these pitfalls by being very narrow. The map consists of Japan, and nothing but Japan, and although there are several expansions set in different time frames, each of the campaigns stays within a reasonable narrow scope. This means the unit roster will not change much, and some may find fault with that. I like it, because it means you can really get a feel for your troops, and they never become useless. There are no large empires to carve out, and this may hold back those who are bent on world domination. But it does make for a much smoother experience gameplay wise, and it is also easier to keep track of things.
The AI usually is the main flaw in Total War games, and Shogun II is not a complete exception. I do feel though that the AI of Shogun II is the best we've seen yet, and all in all Shogun II provides a good challenge even to the veteran Total War player, especially if you choose some of the really difficult factions such as the Tokugawa. That old Togugawa Ieyasu certainly did not get things handed to him on a silver plate!
If there is one thing I would change about this game it is the sudden and anti-climactic Realm Divide. Suddenly age old alliances start to deteriorate, and everyone declares war on you. It feels really metagamey, and makes you wait for ages after you've hit the treshhold, and you know you can't conquer any more provinces or else you have to deal with that annoying Realm Divide. I find that the solution offered in Fall of the Samurai did it much better, as there you are divided between your collective enemies and your allies and friends.
But all in all I can definitely recommend Total War: Shogun II. If you would have to pick up just one Total War game, this would be it.