[thejeff]

It's pretty common here for people to bash the AI, claiming it's completely worthless and doesn't put up any challenge.
And it's both true and not true.

The AI is pretty competent at the things AIs do well. It can usually expand and build up big armies and attack you with them. This can be quite challenging, particularly with higher difficulty AIs and in the early game, where you don't have a lot of flexibility in beating big armies.
The problem is, the AI is basically a one-trick pony. And this is a complex game. There are counters for everything. And counters for the counters. And so on. The AI doesn't look at your strategy and adapt to it. So once you figure out how to beat big armies, you only have to tweak that strategy slightly to beat anything the AI sends at you.

Essentially you beat big armies with meat shields and mage artillery or with SCs. Particularly the latter. Once you can field single units that can't be hurt by sheer numbers, the AI loses.