I just bought a Trek 7200fx after riding a Mongoose 26" for years. I've been surprised at the comparatively rough and vibration-filled ride of the aluminum-framed Trek versus my old bike.
I assume its a combination of the smaller wheels and tires, and the aluminum frame.
I plan to do 80% road and 20% bike trails.. but now I'm wondering how rough those bike trails are going to be on my bad lower back!?