Best RTS I've played in years.
For one thing, this game allows the player to go all the way from the beginnings of mankind all the way to the near future of the Information Age. It's absolutely huge. I prefer playing with the Americans, not just because I am an American, but because of two things...Minutemen and aircraft carriers. But of course, everyone will have a different preference.
There's just nothing better than beginning play with slings and arrows, and ending up being able to launch nuclear ICBM's at your enemies and watching the mushroom clouds form.
The different campaigns are awesome as well, from Alexander the Great to the Cold War, using diplomacy and brute force in equal measure.
If you were an old school fan of the Age of Empires series, and are looking for something even better, play Rise of Nations: Gold Edition.