My preferred commuter is aluminum that tips the scale at around 19 lbs, with fenders, cage, and pedals. I'll take the backpack on this one.
I see a lot of roadies commuting in lycra and wearing backpacks here in the South Bay Area/Peninsula, whereas in Los Angeles, it was more the urban crowd (cutoffs, casual gear, messenger bag or rack + pannier). I am a long-time bicycle commuter and belong to the latter category. I wonder whether these roadies are missing something or I am.