I have to admit I didn't read through the whole thread (5 pages...ugh...), but I wanted to specifically mention that they do makes racks for bikes without rack eyelets. Here's one -
http://www.axiomgear.com/products/ge...iner-disc-dlx/

If you do a search you can find several more options as well.

I prefer a rack. I've done both, and a backpack makes my back sweaty if I'm warm, I don't like carrying something heavy on my back while biking, and if you're riding bent over with curly bars it really doesn't work to have a flat backpack (like with a laptop in it) trying to hold onto your curved back (to be fair, a messenger back might be better).