As the Romani, I rarely use guard mode. In fact I rarely use it at all except with Makedonian phalanxes (but then again I use short_pike so I don't need guard mode for hoplite phalanxes). A while back I did some tests with a friend of mind via multiplayer and found the results to be conclusive: if you place one unit in gm and charge it with another of the exact same unit not in gm, the latter will always win. Units in gm do very little attacking and as a result are much better at dieing than killing. Lighter units or units with deteriorating morale can even rout on contact if they choose to stand still and receive an enemy charge. I have to agree with Caesar on this one--it is better to charge than stand still and receive the enemy charge in most circumstances.

I hope that helps.