My professional opinion on rushing has always been that it needs to be a strategic option, but it can't be the only option. If it can't be done, then you might as well start every game in Castle with 30 villagers and 15 minutes off the clock. If it can be done too easily, then you might as well not have Castle and Imperial Ages and all the cool units and technologies that they offer.
Around the office, we don't rush that often, just because we like the Castle and Imperial units. Lately, we have been trying some more outlandish early attacks. We tend to play 8 player games too, which gives you a lot more options in responding to an early attack.
In a balanced game, a well-executed rush should set an opponent back, particularly if he has made no military to defend (an all too common situation in AOK these days), but it shouldn't decide the game right then and there. Rushes have to be effective if done well, but they have to be counterable.
I'm not saying we nailed the perfect balance in AOK or The Conquerors, but that was our target. I think it has taken some time (and lots of viewing of recorded games) for the rush to become as prevelant as it has been lately. The problem, I believe, is that making early military in AOK is very risky--if you are rushed, you look like a genius, but if your opponent booms, you probably just blew the game. There needs to be more of a middle ground--something we will try to achieve in future titles.