The thing with magnetic (and all) tank bags is that you have to keep the tank clean to avoid scratches.
As for me, I have two -- first is a small Marsee Bullet Bag. It'll hold a digital camera and my sunglass case. It does have a plastic "window" for a note card or directions.
And a basic cargo from Cycle Gear:
No map window, but it holds quite a bit more -- especially with the expansion gusset unzipped.
And neither one covers up the speedometer!