I bought Cossacks II when it first came out. I had a great interest in the Napoleonic history period. I enjoyed the game at the time. However, my advice is:

Pros

Nice historical game.

Can be fun.

Refreshing Napoleonic feel to it.

Cons

If you are going to buy it, get the Stand alone expansion as mentioned above.

Beware it was an old looking game when it came out, and now it still will be.

The battles can get tedious and long winded.

Games like Total War, Civilization 4 and others seems a lot better.

If you really are a fan of the Napoleonic Wars, then also look into Imperial Glory. Whatever people say about it, there is nothing wrong with it. I personally think Imperial Glory has the slight edge over Cossacks II, being a bigger game overall, not to mention having better graphics.

Oh and another thing, Cossacks II has decent multiplayer, though beware the games take a long time!