I have the Nelson-Rigg Silver Streak - has magnets on the bottom of the bag and flaps on each side with additional magnets to keep the bag stable - and it works very well. I also keep the tank well waxed, never set the bag down on the ground (where it might pick up metal shards) and always check the bag bottom just before I install it to ensure that there is nothing on the bag to scratch the paing.
I keep a piece of cardboard stock in the bag - when I take the bag off the bike, I put that cardboard on the bottom of the bag before I fold the side flaps over onto the bottom - the cardboard makes it easier to pull the flaps back off the bottom of the bag - without it the magnets on the flaps stick to the magnets on the bottom of the bag so tightly that it looks like the bottom of the bag is going to tear when I pull the flap magnets off the bottom.