+1 on panniers.
If, however, you insist on backpack, I was going to suggest a suitably sized Deuter. I see you went that way already. We have one of their middle range backpacks (maybe 40-50L of volume (1)) and the frame does a reasonably good job keeping the bag off your skin and providing space for ventilation. It seems the frame is not used in their smaller backpacks, at least not in the models we saw here.
(1) As a side note, has anyone else noticed the hiking bag capacity claims are ridiculously inflated? I was in search for a large hip pack and saw an Osprey Talon 8 in a web shop, advertised as 8 liters. Went out of my way to actually see one in a store, and a good thing I did. The bag was barely the size of three 1,5L PET bottles in total (happened to have one with me). I showed it to the doubtful store clerk and asked why they had BS descriptions in their web shop. He admitted the capacity was way short of 8L, but "that's the manufacturer's specs, so we're good". Ummm... legally speaking no, you're not.
Yeah, I want a backpack because I do bike-train-bike to get to work, which involves going up and down stairs like 5 times--the added weight to the bike itself would be a major hassle, as I've ascertained by carrying my girlfriend's heavy Giant bike up just one flight of stairs. A hybrid thing might be an option, but at this point I think a backpack just works better for me.
Originally Posted by Juha
I have the Deuter Futura 42L with the frame, gave it a shot commuting the other day and found that the top of the pack would bump against my helmet when I was riding. Also wasn't sure what to do with the massive hip belt. These things don't bother you?
I haven't tried Deuter on bike, just hiking and XC skiing. Can't you just keep the hip belt open? Tuck the loose ends somehow, so they don't get tangled with anything? Not sure if you can do anything about the top bumping your helmet though.
Deuter makes biking specific backpacks, but I'm not sure they're large enough... see the Cross Air for example. http://deuter.com/en/DE/product/1-76...sAir20EXP.html
Have you tried or heard of Road Runner Bags? They make a bag that could be up your alley. Roll top, waterproof, water bottle pocket, reflective strip, u-lock holder, etc. Handmade out in La. For cycling backpacks they do a really nice job. You can check it out here: http://roadrunnerbags.us/ Attachment 346384