I've ridden mostly steel and carbon fiber bikes. My experience on one aluminum road bike was harsh and nasty. It was most likely due more to the tires and the way the frame was built than the material used. My modern TIG-welded 853 steel road bike rides very similarly, although it's about a pound heavier than the aluminum. It seems that designers of aluminum bikes tend toward the light, racy and harsh-riding even though they don't necessarily have to.