Put the geared bike back on duty this AM after my knees started griping about my single-speed hammering. Knees today feel fine, just a faint "funny" sensation, no pain. Whew, didn't stay in denial long enough to really mess myself up. After having gotten used to the more aggressive geometry of the SS, my handlebars feel a bit too high. Going to take them down and forward a notch or two (adjustable stem) and see how it feels.
Definitely finding I like the feel of steel better than alu.