I'm working in Germany this month and there are thousands of commuter cyclists riding around. Every form of commuter bag that can be imagined. What I figured out with my own commute is that I didn't want to have a bag that couldn't also be the one I took off my bike and carried into the office. That's one reason I sprang for the Arkel commuter. Yes, it's too much money but it works great and serves as my shoulder bag when I'm off the bike.

Buy the bag(s) that will attach firmly, has a good rain bonnet and will hold what you regularly take with you to work: an extra sweater, a lunch, your purse, whatever. Then it is simple to just leave it on the bike and drop your customary stuff into it daily. Check out these forums for all that attachment, testing and reviews - then get one that serves, works and hopefully satisfies any expected fashion statement. Just like bikes, there are those that look really cool but aren't very dependable or well-designed. Commuters on this forum ride with every imaginable pannier, bag, milk crate, basket, etc. Whatever works for you and your budget...

FWIW, I now ride a Surly CrossCheck with an Xtracycle attachment on it and carry everything & anything I need or want...the bike is strange enough and big enough that drivers along my customary route are used to seeing me and hopefully watching out for me.