Since I started bike commuting, I've gone back and forth between using a backpack or rack/panniers to haul my stuff. I first rode with an old backpack I had lying around. After I got my first decent bike, the Marin Muirwoods, someone suggested getting a rack and panniers, but those just looked...nerdy/dorky. Which usually isn't a problem but seemed to project a bicycling image that ol' self-conscious me wasn't comfortable with, that of a serious cyclist.
Then I finally broke down and got a Topeak rack and trunk bag with zip-down panniers bags. It was nice to get the weight off my back, and I could haul more stuff with the panniers. The bike handled a little differently with the loaded panniers, but I got used to it.
But it can be a pain to ride somewhere and then deal with taking the panniers off the bike and carrying them around. Much easier with a backpack to just get off the bike. So after awhile I went back to the backpack.
I used the backpack all summer long in 2012, including some long after work rides with a U-lock in the backpack, and it never bothered me. But this past summer I got tired of carrying the heavy, sweaty backpack and bought some nice Ortleib panniers. Ahhh, felt good. Except sometimes I'd get halfway to work and suddenly panic thinking I forgot my backpack.
The only problem I have with the Ortleibs is there isn't an easily accessible pocket on the outside. Yep, I'm one of those guys. Sure, you can stuff a lot in the roll-top panniers, but is it asking too much to not have to pay $30 bucks more for an outside pocket accessory to stuff your wallet, phone and keys in? No, you have to unbuckle the bag, unroll it and dig around in the bottom of the bag to get to your loose items. There is a zippered pocket at the inside bottom of the bag, but it's still a pain to get to.
But now with winter I've gone back to the backpack again. I'm alternating between riding the Vaya and a studded tire mountain bike without a rack, so it's easier to stick with just one hauling stuff mode. Makes it easier in the morning to always use the backpack instead of switching between the backpack and Ortleibs depending on what bike I'm riding.
There's been a couple times this winter when I wished I had my panniers, as they're easier to fit heavy or bulky items in if I've stopped at the store on the way home. Nothing like stuffing a nice flower arrangement for the Xmas dinner table in the backpack and hoping it looks nice by the time I get home. Luckily, Boehm's wraps up the flowers expertly for me.