I haven't picked it up yet, I won't have any time to play for the next few weeks so I'm just waiting to get it for my birthday. I did play the demo, though, which featured both a land and naval battle, both of which were exquisite and very fun.
I can say more about the Total War franchise as a whole, however, as I've been playing since the original Medieval came out back in 2002. The series combines a campaign map, where you are able to build up the infrastructure and trade of your empire, engage in diplomacy, and conquer territory, with the ability to play real time battles on a massive scale. Both sides are very fun and designed to make you feel like you're actually fighting in a real-world situation, especially the land battles, which are won through real tactics and more by breaking morale than necessarily killing. There have been a lot of changes in the franchise, of course. The first two games, Shogun and Medieval, used a Risk-style campaign map where you could only move armies from province to province and battles only featured sprites to represent the soldiers. It got a massive upgrade for Rome when they switched to more of a Civ-style map where you can move your armies around the map and take advantage of terrain more easily. The units were also finally rendered in 3D. Medieval II took Rome's upgrades and refined it and, in my opinion, was much more entertaining than Rome ever was. Empire appears to be in the same vein as the last two but is making a lot of big upgrades in some areas, it would really probably be best just to read up on the features.
By the way, the demo is available through Steam if you're interested. It's a pity they don't have a campaign demo, but the battles are entertaining.