Johnny Pain, author of the GreySkull LP program, does not recommend doing assisted chin-ups to build up to unassisted ones, claiming all they'll make you good at is assisted chin-ups. For instance, on his forums (login required), he has mentioned in the past:
Yes, ditch the band assisted chins lest you become extremely good at doing band assisted chins and little else.
Band assisted chins are good for one thing, making you real good at doing band assisted chins.
If you're in a gym that has an assisted pullup machine, they probably have a pulldown setup. I like V handle pulldowns heavy and with good technique.
So he recommends v-grip pulldowns and chin-up negatives spread through-out the day, instead of assisted chin-ups. Is there any reasoning behind this?