I find the AI in AoE2 to be fairly challenging, such that it makes the game more fun. The AoE3 AI, on the other hand, I find to be pretty poor - I almost always win on Hard (whereas in AoE2, Moderate is a good challenge), and only Very Hard consistently beats me - IIRC the only difference from Hard is that the AI gets a 100% production bonus, so it doesn't feel like the AI has actually earned its victory.
For multiplayer, it is too much of a clickfest for me. I only ever played AoE3 online, and while it was fun at first, it became increasingly boring as time went on, and I don't think that I've played a single game of it in the past year.