My primary bike for commuting and sporting was a 40 lb Columbia one-speed w/ a coaster-brake rear hub and mtb-like bars. I took particular delight in pounding it up hills right past the folks clipped into their "lightweight" steeds (who would then pass me on the downhill). I once took it on a 50-mile relatively hilly club ride and had no problems keeping up with the pack. The frame geometry was just perfect for me, particularly in the length of the chainstays and the ways I was positioned in relation to the rear hub. I could stand on that bike and really go (of course, that was over 10 years ago; don't think I'm nearly as strong now).

However, as the owner of several 531 bikes, I have to say the combination of relatively lightweight steel and lack of flex is ideal. When I pedal hard, I want to feel like that energy is being transferred to the drivetrain without undue slop. 531 frames give me that sense. Now, I've rarely ridden non-steel frames, so ymmv.

Neal