Just plain great.
In the campaign, you start out being able to control one of three Roman families, each controlling a mere 2 territories placed to show you where your main conquest missions will be. The Senate will constantly be bombarding you with missions to do anything from conquer a city to obtaining trade agreements. Howeer, you could easily choose to ignore these missions, as they only offer rewards, no punishments. However, as you gain more and more territory, the People will like you more, while the senate, after a point, will like you less. This is when they start punishing you, occasionally even telling you to have your faction leader commit suicide for 'the good of the Republic.' It is also around this point that you will gain enough popular support to attack your Roman allies and start a Roman Empire. Once you conquer 50 territories and Rome, you win.
However, this is not the end of the campaigns. In fact, it is only the beginning. After you play through once, the game unlocks most of the other factions so now you can play as one of 12 or 15 factions, with the same objective though. Besides the campaign, there are battles. You can battle in the campaign, or historical battles, or you can even create your own battle from a multitude of available maps and all factions, playable and non-playable.
In the end, Rome :Total War is a game with high replay value, and will continue to be played by many for years to come.