The SM57 will give credible results on any singer, as long as the "engineer" is up to the task. A combination of good mic placement, and judicious use of EQ
leads to a high quality recorded vocal. They take such tweaking amazing well.
It's an important microphone to try out, on many singers it will rival
expensive Neumann's, as long as the mic pre is a good one.
(I fall into this catagory, especially on my Shure 546)
It's fraternal twin, the 545 series, is comparable overall.
The 546 and SM56 were similar to the SM7 in having selected capsules
and were marketed to the pro studio community that way.
When you look for either one, look for the old ones, "Made in the USA" that will say "Unidyne III" on them. They are more consistant in tone quality than the ones made in Mexico, although they can sound excellent too.
Chris