I have four, and they are not really ok for an acoustic. They are a lot more tinny than a SM58, bass response is not great. They really cut through the mix, but you have to work the EQ a little. I would not recommend them for an acoustic. An electric though, I have no experience with.
If you dont have phantom power, the EV co4 is affordable and better for acoustic than the sm57. But the 57 is no slouch but twice the price.
If you do have phantom power get the mxl603s, a little more will get yoy a studio projects B1, Ive heard it get good results on acoustics, vocals, but I would rather use a dynamic on amps.