Napoleon Total War is really an Expansion for Empire Total War. Don't buy it if you have Empire Total War.
It has the same engine and almost the same features and game play.
They made it a stand alone game so they can charge more money for it.
If you have Empire Total War then there is no need to buy it.
If you don't, then you may want to buy Empire Total War instead at a cheaper price.
The game also has many issues.
You will need top of the line video card to run this game on high settings.
Load time takes a long long time.
The Battle AI is weak, too easy to beat and it has many bugs.
Siege battles are broken.
There are many path finding issues during real time battles.
Even the Campaign AI is not that good.
Sea battles are tedious and boring.
The campaign game play gets boring after awhile with the continues need to upgarde every single building again and again.
You never have enough money for upgrades.
Personally I like Medieval 2 Total War much better.