The Nashville Number System is faster to write down as you listen to a tune, has methods of indicating ryhthm associated with changes (underline means half a bar, diamond around a number means to play the note once and let it ring for a whole bar). It's just better all around for popular music. Roman Numerals are really only used in analyzing classical music as far as I know.

Part of my education, in commercial ear training class, we had to write number charts and get it right the first time through. It sounds hard, but with practice it's really quick.