Ventura Luggage
I've got a set for the GS. Great capacity and easy to use. Also keeps the weight of the kit over the centre of the bike. You will have to buy a set of rails specific for the 650 to mount the frame which carries the pack(s). You shouldnt have to take the BMW grab rail off. I didnt for the 1150 but the 650 might be different. If using only one bag you can carry it either on the back seat, or over the back of the bike. Two bags and there ain't room for a pillion. The bags are good quality but not waterproof, although shower proof.
Mike