i haven't got any experience with either MD421 or e609. heard good things about them though. i've found a couple of samples where some guy made direct comparison (no eq, same placement, yadda) and out of the three i actually preferred the e609 - it had a nice balanced sound and a smooth bottom end. the sm 57 wasn't very good (strong mids, somewhat "closed" sound, hard to describe) and the md421, though somewhat similar to the e609, was a bit too "boomy" for my liking.
however, the comparison, although fair because of the same placement, also isn't exactly valid if you ask me - for that very same reason... all the mics were pointed at the middle of the cone, on-axis. i hear sm57 is very picky when it comes to placement (i can attest to that, heh), while the e609 will do the trick mostly anywhere (or so they say).
while we're at it... does anyone have some helpful tips on the placement of the sm57? i like to put it on-axis somewhere between the edge and the center of the cone and i dig the sound, but it's still not quite there... of course it might be my amp which i've adjusted based on what i hear when i'm sitting on the chair, not what the mic hears. still, if anyone has experience, well, uhm... please..
also, does anyone know where could i get some samples of a distorted guitar recorded with a condenser mic? i searched but found nothing. i wonder if anyone does it...