I'll admit, I have had the cavalry problem mentioned by XCrusaderguy01 happen quite a few times (my computer seems to carry some hidden hatred of me, it always happens at the worst times). However, on most occasions it seems to work perfectly fine (i.e. mass slaughter of anything that dares to stand in the way of the iron-shod hooves of my glorious horsemen.)
Anyway, Medieval II is fun. The expansion adds 4 campaigns, each almost acting as a completely different game, I might add.