I have an Icon Primer Tankbag which is great, it is reasonably sized but not overbearing like other I have seen.
I can fit my house keys a can of drink or two, a sandwich, disc lock, gloves & my mobile phone in there comfrotably. I was getting the shits with carrying my backpack everywhere for nothing & really don't like stuff in my pockets when riding.
It is held onto the tank by magnets & it is very stable at speeds.
Just thought I would recommend it for anyone in the market, for $80.00 it was a good investment.