We have the Arakawa system, and my experience is like Tony's. The frames' tops tilt out from the wall, especially if you hang more than one frame per wire (we have 12 ft. ceilings here). One fix is to put a staple in the wall near the hanging point -- but that messes up the wall.
We still have the system in place, but if I were doing it again, I would choose a rigid support such as hanging rods. I've heard good things about the system United offers, but I didn't know about it when I bought Arakawa.
Any hanging system is better than perforating the walls with hooks, I think. It looks professional and makes for easy re-hanging, too.