For a game that came out in 1998 this holds up extremely well now. I finished all the campaigns at when it originally came out, and had no problems with motivation to finish it a second time, the game is just that good.
The learning curve is excellent, teaching you how the units work in a manner that is not overwhelming and the strategies you need to apply in the later campaigns in order to win are masterful. It does have a few minor flaws in my opinion, there are a couple units that seem particularly useless (such as the non-ram siege units), and the AI can be a little strange at times (the enemy will march units right past you without attacking for instance).
Overall though, this game is one of the best RTS’s I’ve ever played and I would dearly love to see a sequel that kept to the same formula (as opposed to the less critically acclaimed Age of Empires 3)