I commute with a pack. I haven't found it makes that much of a difference, honestly, I suppose if it was off-balance on my back or filled with awkward bulky things that poke me, maybe...

But then I'm pretty much used to doing lots of things with a pack. A well-adjusted one won't really even feel like it's there, I mean it will add weight, but it won't throw your balance off that much, especially if you're just doing normal road riding.

It also helps to have a pack with hip and/or chest straps so that you can keep the pack close to your core and keep from feeling like it's pulling you off the bike.

I think if someone is a timid rider who is unsure of their balance to begin with on a road bike, putting a pack into the mix is a recipe for disaster. If you're comfortable on your bike, it shouldn't make a big difference.

K.