I got tired of taking >25lbs of saddlebags every day to work, just so I could carry a water bottle. I bought an Icon primer pack tankbag/backpack. It kicks ass.
you don't notice it's there at all while riding. When I get off the bike and wanna walk around (like riding to a public event") I can clip in the helmet, stash my gloves in the pack, and it's all on my shoulders. For $50 or less, it's a must have.
<span style='font-size:21pt;line-height:100%'>IMPORTANT WARNING</span>
I wouldn't advise using magnetically mounted tankbags with hard drive based mp3 players such as the ipod.
'nuff said.