You get lots of opinions here and at the other BMW sites.....one bag that has a unique design, is easy to use, is easy to deal with when fueling your bike and is super easy to remove to take with you..is the Big-Mak Tank Bag.
There are various sizes and attachments that you can choose from to make the bag even more user friendly.
I see a tankbag first for ease of removal when I re-fuel...what do I do with it when I remove it, does it flop down, do the straps get in the way and what does it do to my tank during use.
I carry many riders goodies, pens and markers, tire guage, heavy SLR camera and use the accessorie water bootle holder and the bag never sags to touch my tank.
I've moved this bag from my '02RT to my '04RT to my '05RT and now over to my '06GT...so it's really a keeper.