I use one pannier on the left because I like not having a backpack, but I get other commuters blowing by me everyday and they're all using backpacks. Granted, most of them are on road bikes, and I'm rocking the Navaro Fusion , but there's plenty of people on other hybrids or mountain bikes just zipping right by me.

Now, I have no qualms about being passed - does't hurt my ego or anything. But my 10 mile commute takes me between 50 - 65 minutes depending on wind conditions, and it would really help my daily routine if I could get that time down in the 30 - 40 minute range.

Obviously, I don't know what other riders carry in their packs. I'm carrying a big old IBM Think Pad that weighs a ton. But I left it at work yesterday for the ride home, and I was only a little bit faster, not a lot.