With magnet bags, always be sure the tank and bag's bottom are clean. Place the bag straight down and pull it straight up so the magnets don't move on the tank. That'll go a LONG ways towards minimizing scratching.
My son purchased a Rapid Transit Recon 19 bag last summer and the zipper broke on it. He took it back to the dealer where he got it and they gave him a new one - this was just a few days ago.
He likes the way the bag is set up, but I'm a little iffy if this is an indicator of their quality. My Joe Rocket bag is now ~7 years old and is still holding up just fine.