I play on the Harder levels of the game and find it to be a reasonable challenge. Certainly, in certain situations, the AI is a little weak, particularly when the player defends against their attack.
I must admit, I did give the game a break for a while after I first got it as it did seem a trifle repetitive. I think it was the end goal of simply conquering every province, you know. That doesn't appeal to me but the game is still a very good technical achievement in my book.
I like the depth and the balance of RTS battles matched with the strategic elements of the map - building settlements up, army customisation etc..