I'm surprised nobody has mentioned the Lone Rider panniers. I have tried the Bumot soft panniers but they are not waterproof so you have to pack everything in one of their watertight inner bags which dont really fit the shape of the panniers well so you lose some storage space. In contrast the Lone Rider panniers are completely waterproof so you can just throw stuff in them like a hard pannier. They also have (removable) liners to make sure they keep their box shape and they fit directly on to the existing GSA pannier frame. In addition they can be locked onto the frame and locked themselves with a combination padlock. The only PITA for me is that they are a bit of faff to close because you have to roll the top, attach some straps and then close the locking frame if you want to padlock it. All in all, not as easy to use as hard panniers but the vital bit is that you can drop the bike without damaging them, unlike the standard BMW hard panniers which are far too easy to bend out of shape