You play the campaign seriously, but freely you play... not so seriously
In the Phase Creation you will not get diverted, or something like that: It's too boring. You need to program everything, and then you just know what will happen before. It'll be fun if you do it with a friend, you do the map and he plays, but surely you will not want that.
Age of Mythology increases something cool to the "Age Of" series: the mythology. You will now have an army of Egyptian Zombies, or Chimeras, or even a Phoenix to play, not only that "boring" soldiers.
If you are playing this type of game at the first time, i do not recommend this, because you will surely be confused. The first impression is not good for the ones who never played this type of game.
In resume, I can say that: if you are going to play, have a friend to play on line, because if you do not have, the Campaign will be the only good thing here.