I've been looking for a comfortable pack for awhile. I currently have an older Axio Hard Pack. Nice pack but not comfortable for daily commuting use on my Thrux.
Have tried a variety of different packs borrowed from friends. The most comfortable one I've found that will hold all my needed equipment (Jacket, Gloves, Chaps) and still have the capacity to get a little more in is the Maxpedition Monsoon Gearslinger
The reason I find this the most comfortable is that it is a "cross back" carry (i.e. single shoulder strap). With this design the large section of the pack rests on your lower back and when on the bike it rests on the seat. It tapers up to the top of the pack where the strap goes over your shoulder... this positions the pack "off center" and doesn't interfere with your helmet, you have full range movement of your head without hitting the pack.
It is also very easy to access as being a single shoulder strap you just "swing" the pack around to your front to get into it. The "side" of the pack where it comes around the front has a small pouch for quick excess of cell phone and such and also webbing to attach other pouches.
I'm hoping there is one under the Tree on the 25th as I've been hinting for a while.