It greatly depends on what you're using it for. I have a real old SM57 that I use on a number of different things. It may not be the BEST at anything, but it just doesn't flat out suck at almost anything either. I'm a drummer, so I've used it on snare and on hi-hats. I currently use
a Sennheiser e604 on snare 'cause it stays out of my way better and it gives me a really good signal. I'm using the SM57 on hi-hat right now, but I switch off to a Senheisser e835 very often because it's a bit brighter.
Whenever I do a speaking engagement, (I'm a professor and give talks and lectures a lot) I use the 57.
It's a good vocal mic for live gigs, and will stand up to showy singers that tend to abuse mics on stage.
It works okay on acoustic guitar in small clubs.
There are a lot of similar mics to it out there, but they tend to be "clunkier" and more fragile. It's good at many things, great at nothing, and bad at...nothing I can think of. Well, maybe not to record a lead voice track, for background vocals it'd be okay.
In the Shure line, the SM 7 would be the dynamic mic that I would want to us for a lead vocal. But I would probably go with any of a number of good LDC's instead.
I have an old AKG D690 that I got from a friend that'll give the SM57 a run for it's money and it gives a better signal on lower frequencies too. But it's "clunkier".