Living in a subtropical climate (similar to Florida), I like my rack and panniers for commuting. I'm totally fine with backpacks for mountain biking (my cycling background) as there's nothing to obstruct the handling of the bike. This summer I tried commuting with my backpack and, although very doable, it is horrible arriving at my destination with a VERY sweaty back - even on short commutes. Whenever I had to carry the 14" laptop, textbook and a cablelock, it was starting to become uncomfortable on my back. I just can't imagine if one had a bad back! I haven't tried the messenger bags, but I love panniers so much because there's no weight anywhere on my body and it keeps the bike quite maneuverable even on busy roads (unless you treat every ride like a race). And one more thing... I feel it adds a seriousness of purpose in the eyes of drivers and pedestrians... this ain't no mountain biker, race nut or rushing-to-make-a-delivery messenger guy. A bicyclist with panniers looks like s/he has important things to do, including, perhaps, saving our planet. I feel that perception is priceless!