In order to most effectively use the Strap-Surfer, you need a bag strap without any padding on it; for instance, the neck pad typical on a messenger bag's strap will interfere with the Strap-Surfer's ability to slide up unimpeded if you've got the bag against your lower back. Backpack straps are generally too thick.
It's also tricky to set the strap length appropriately. Too long and it dangles farther than I'm comfortable with; too short and there isn't enough give to bring a camera up to my eye without dragging the weight of the bag up with it or to stash the camera inside the bag. On the other hand, the weight of the bag adds a stabilizing counterbalance to reduce camera shake.
For people who carry around a standalone lens bag, the Strap-Surfer might be a nice option if you'd prefer not to load your neck up with straps.