I've been a fan ever since the first Shogun even though I can barely remember playing it - was that long ago.
The AI - uber challenging. Diplomacy is essential and cannot be ignored unless you want to have swarms of AI storming your castles every turn. They prey on the weak at their weakest points.
The design, the music, the art - a lot of care and attention has gone into this. I've only got about 15 hours and into a 2nd campaign after being handed my arse on the first for the very first time in a TW. Some discontent about what happens when you start dominating the map called a 'realm divide' that I've yet to experience. A mod is already in beta to balance that aspect.
If you're a fan, get it today. If you've got a whole bundle of games already in the queue (like I currently do...) wait for a special.