I used to commute with a backpack or messenger bag. I have a rack and panniers on my new dedicated commuter and much prefer it. I can carry a lot more (groceries for instance) and I no longer have to worry about a sweaty back. As others have said, it doesn't really impact the handling at all. Once you go rack, you never go back (in all seriousness, I was resistant for a while, until I used my fiancee's bike with a rack and panniers and realized how much it sucked to have a bag on my bag...if necessary, I just bungie my bag onto the rack and even converted an old messenger bag into a pannier).