For my money, this is the finest strategy game on the market. The balance of the various kingdoms is far better than in the original, with all empires having serious strengths and equally devastating weaknesses. One must learn the value of combining force types into effective mixtures of defense and offensive units to even have a chance at success. The serious player will probably take something on the order of a year to master the game, because the complexity level is so high.
The campaigns on this sequel are of a much higher caliber than those on the original, so single-player use is very much improved. In fact, the campaigns are actually historically accurate, unlike those in Age of Empires. Indeed, the historical section on the CD is fascinating reading in and of itself, and will please those who love the time period, as well as the tactical problems involved.

Age of Empire's real strength, though, is like that of its predecessor: the game's extreme playability over network. I heard that Age of Empires was the most popular network game ever, and there's no reason Age of Empires won't follow in its footsteps. This is GREAT stuff to play in real time with others. Be warned, though: players in Microsoft's Gaming Room are ruthless to the novice. Don't even think about going online until you've gained a level of proficiency playing by your own. And take the time to learn the keyboard shortcuts: you'll need the time it saves.

For most games, I think that strategy guides are a wimp's way out, but with AOE, I highly recommend the Ensemble Studios' guide. It's invaluable to improving the quality of your game-playing experience.