I have a Cortech 10 liter tank bag with magnetic mounting (very strong magnets grip the tank, heavy cushioning prevents paint damage). I carry a water bottle, maps, bungie cords, tire pressure gauge, flashlight, and so on. It is not big enough for large items.
Smaller than one in photo above. Looks sharp on my white CB500F.
One issue with tank bags on our bikes - you have to remove them to access the fuel filler cap. Takes two seconds with magnetic attachment. Bag has carrying handle.
The bag is not completely waterproof, OK for a while in very light rain. Comes with a rain cover which you have to put on.
2014 CB500F: GSXR600 rear shock, Traxxion fork emulators+springs, Kuryakyn Throttle Boss, IDEA Gear Indicator, Slipstreamer S-06 windscreen, SW-Motech handlebar risers, LED tail light, rubber tank side pads.