The only time I use groups now is for moving the whole formation around. Once the actual fighting starts, I dismiss the group, as the unit AI seems to be more weighted towards keeping formation than actually responding to orders.

Even worse, I had one time a group of infantry, and a group of cavalry. I ordered the infantry to attack a group of enemy infantry head on, and ordered the cavalry round the back, so I could take 'em in the rear. Instead my infantry group started to move round the back and the cavalry went for the head on charge. No matter how many times I re-issued orders! It was like the two groups would only obey the other's orders. Gave up on groups after that