I can recommend the Famsa, had the same bag on 2 diff 12's now and it's a treat. The zip is fiddly at first, but you soon get the knack. I got mine from
Have to agree the BMW large bag is very good.... And not too big, if that makes sense. I went through some serious rain in Italy last year and the whilst not advertised as waterproof it did keep my kit dry...
sw-motech does a very nice tankbag, that's supposed to slide on their tankring (bought seperately). Bit expensive, but the tankbag is easily fixed on the bike, and there's no scratching the tank. When you take the bag off, the tankring remains on the bike. There's a standard tankbag (in different sizes) and they also sell 'electric tankbags', a system to supply power to the tankbag, handy for charging stuff. Have a look on their website.