+1 on the panniers.

I've tried it to see how it felt and more than about 15 lbs is annoying (at least for me) in a backpack (I use a chrome backbone), and I only commute 7.5 miles each way. My normal load is around 10 lbs.

I used to use ortlieb panniers but got tired of removing them and the rack when I wanted to do a recreational ride. Plus they made the bike feel more unstable when loaded. But if you have to carry a lot of weight panniers are the way to go since you don't want that on your back.