With all due respect, the "only good on certain voices" comment isn't
quite accurate. The SM57 is one of the standard microphones used
for live and studio work that does a credible job (or better) in recording
most voices. The 57's sound isn't really "natural" either, in the sense that
it has a signature tone to it that's very well suited for blues and rock.
It has a massive presence peak that helps cut through a busy mix.
Earthworks, Schoeps, and DPA would be examples of microphones favored
in classical recordings due to their more neutral/transparent sound.
The SM57 is very sensitive to what microphone pre you use BTW.
It's been unfairly evaluated by many home recordists because it's
usually plugged into something like a portastudio pre's or a Mackie mixer,
and can't reveal how good it can sound. Another factor is that the top
recording guns usually EQ the '57 with a high end parametric equalizer,
not a Behringer, on vocals.
Chris