I've used them a lot for acoustic guitar. I like them. For a stereo setup, I like to place one about 8" out from the 10th fret pointed slightly in toward the sound hole, and the second about 5" to the right of the guitarist's right ear (assuming they're right-handed) pointed down toward the body of the guitar. This gives a nice image and a good balance between the woody/boomy body and the tinny/finger-soundy neck. For mono you just have to move the mic around until you find a good spot for that guitar and that room. Anyway, I hope that helps, good luck!
-Peter