Impressions:
1. Heeeheeeeheeeeheeee.... this is VERY entertaining. It feels like a super powered version of Napoleon.... except the swords are still very viable and very scary. I laughed when the AI charged me with samurai.... I wasn't laughing when they reached me.
2. Battle numbers are pretty insane. Definitely the largest battles in any Total War game. It's retroactive with all Shogun 2 versions too, so even those without this expansion can still run 40,000 man battles.
3. Naval bombardment is all kinds of awesome. You get more / get better bombardments depending on the number and quality of ships you have nearby. The AI knows to avoid it too (you launch a flare at the location), so you can't just shoot it at some people marching somewhere. Also, it's absolutely hilarious using a giant ironclad to ram a tiny wooden paddleboat in half.
4. Manual control is pretty entertaining. I think it's more of a minigame than anything you actually need to do to win. It's there if you don't have anything to do for a bit in battle. Pretty sure the AI is better than me at aiming anyway.
Sometimes it just feels really good to land a sweet ass cannonball on the enemy general's head.
5. The AI definitely got buffed. It's even more frightening than it was before... and honestly it was already pretty frightening. Hell, I'm still more afraid of the AI than I am of regular human players online (who are dumber and more cowardly than a pool of rectal fluid).
That's my impressions so far. I've done four of the historical missions (which have quite a lot of content to them now, it feels like an entire campaign) and only just started an actual campaign. So I haven't seen much.