I started with the Shure PG58 and most recently acquired the Shure SM58.  I'd probably be all too ready to agree with those who argue that a Neumann or Telefunken mic costing 5 G's simply isn't that much better than a Nady mic costing 10 bucks.  But with respect to the Shure, I'm less sure that the SM58 isn't worth 40% more than the PG58.  Perhaps if you have a sufficiently high-powered and "clean" amplifier and have tweaked the EQ settings to favor the frequencies of the human voice, the two mics could be mistaken for one another.  It was my attempts to occasionally use an instrument amplifier as an emergency P.A. that led me to realize the PG58 sounded somewhat muted and dull compared to the SM58.  Switching to the more common, admittedly more expensive mic, gave me just the extra gain I needed along with added crispness and clarity to the voice (at least for announcements).