This round is like the baseball player that doesn't look like much on paper but seems to hit the clutch home run every time.
This round blends in the crowd--so much like the .308, 270, .303, 7mm-08, etc., yet it's almost completely overlooked by all but the most serious hunters and hobbyists.
It has been described as a "ballistician's dream." Don't let some of the published velocities fool you--much of the commercial ammo is loaded for the original Spanish Mausers that require much lower pressures. If you have a gun made in the 1890s and beyond (check with a smith to be sure) you can use ammo that is on par with any common centerfire fire rifle, with incredible trajectory and stopping power.
During a summer deer cull, I shot a deer in a dead run at just under 300 yards with my Venezuelan FN-49 with peep sights. Double lunged it. Quick kill. Was it luck? Skill? The round? The proportion of skill and luck is debatable (pretty damn lucky I think) but the 7x57 was the constant in this equation.
My experiment may be inconclusive, but a few noteworthy particular big game hunters have definitively proven the effectiveness of this round. Karamojo Bell, anyone?
PS: The 7x57 is the reason we have the .30-06. Look up "San Juan Hill" for more on that subject.