I think AoE3 is really good, but more importantly it HAS (not past tense) the potential to be great.
When you play it, you get the feeling that it was not quite finished when shipped. It's a well-known fact that ES would have liked to add more features but essentially ran out of time, making last-minute changes right up to the end. This is obvious because some aspects of the game, such as the graphics, physics, and card-playing strategy are beyond top-notch for this genre.
Yet, other aspects that would seem to be essential for an RTS released in 2005 were left out. Althougth economic micromanagement has been improved, the game currently lacks anything important to do with that extra time, as meaningful battlefield tactics are found conspicuoulsy missing. But, the game has defend, cover, and trample stances...so you know there was an attempt made at tactics; it just never got to a finished point.
It's frustrating for now because it's so close to being great. With relatively minor effort, ES can turn this into a rock-solid classic.
If you are just looking for a fun game, definately pick this up. It IS very fun. (I still smile every time a villager takes a cannonball to the kidney and goes flying head over heels) However if, like many people, you are short on cash and will probably only buy one or two games this year, I might wait a bit and see what ES does with the game in the next couple of patches.