Krystof01,
If you look at the lists of mics available at major studios, many of which are online now. You will find lots of SM57s and not a few SM58s on their lists.
The SM58s are very neutral and not as "in your face" as a vocal condenser mic, but it depends on the effect you are trying to acheive. It is the most popular live vocal mic, and both are very, very good dynamic mics for the price (new $77 and $100 respectively on the web, last I checked).
The 57 and 58 have the same assembly--the 58 has additional pop filtering under the ball that allows it to be used at close proximity such as in live applications. It also seems to make it less sensitive for instruments. I've used the 57 with a pop filter (pantyhose) or aimed a little off axis and it makes a great recording mic. At the time, it was the only real mic we had, and I am still very proud of those recordings and they are more than demo quality (after a little signal processing). Just keep the singer a foot or so away from it. Don't forget that some of the highest end recordings with the least processing (classical, jazz)are done with ribbon mics, which are a type of dynamic mic. Dynamics are the preferred mic for blues harmonica.
A long winded way of saying--if you've got an SM57, go for it.
There are many people on this BBS who are very happy with the NT-1. But I don't think you need a large diaphragm condenser mic for people to take your music seriously.
Mark W