I just bought a set of Snap-on metric ratcheting open end/box end wrenches with a big question mark over my head. After using them a few times, I already am loving them.
I did a bit of "testing" by hooking the open end on a nut that would have taken alot of torque to remove and to my surprise, the open end held on much better than I had expected. I did not break the super-tight nut loose because I already was applying significantly more force than what I would have been comfortable with. I was amazed.
However, I wouldn't recommend them for breaking or tighten fasteners though. Use the standard wrench (or socket) for that, and use the ratcheting combo afterward.