I commute daily (40 min to 1 1/2 hrs, depending on how adventurous I'm feeling) with a small backpack, and as long as it's not too hot I barely notice it's there. It's actually a add-on daypack to a larger Osprey hiking pack, weighs nothing, is very narrow so it sits nicely between my shoulder blades, and does not have a padded hip or chest strap. Great for carrying lunch, keys, wallet and a few items of clothing so my colleagues don't have to see me wear the same thing EVERY day.

Anyhoo, I use it on road rides as well and have never really understood what roadies have against them. On fast training rides I'd rather move (and sweat) freely without a pack, but otherwise it sits just fine.