I hope to find out when I either finish my homemade rack or decide to pony up big bucks for a pre-made rack. I like that Topeak rack and basket that joyota posted above, but my local REI wants a fortune for the pair. I might end up with a milk crate instead, but I digress...

My biggest gripe about a backpack is the discomfort but also the sweat it produces. One day two weeks ago I brought an entire set of clothing the day before and then rode to work the next day with no pack at all. My shirt was almost dry when I arrived. But, with the pack, I could probably literally wring sweat out of my shirt from the huge wet spot on my back where the pack sits.

I realize this has nothing to do with "fastest" at a glance, but my thinking is that a more comfortable rider is a faster rider, all other things being equal.