i learned this in college but have also experienced it in practice too.
when my band are recording just demo ideas sean oursinger uses a 57 and we d.i the guitar and we get just demoish sounding tunes,
whenn we use a proper studio sean uses a rhode nt2
and all the guitars are mic'd up properly with microphones with a tailored frequency response and it makes all the difference.
check out
www.myspace.com/lifelinerock our emo punk may not be your thing but you can hear the difference between the demo tracks (Orca, Ashes, Good Secret) and the final tracks (censored networks, heartburn, Atom Bomb) as we have a mix of both up at the moment and the difference proper mics can make is apparent