demitriden.... generic Adderall is available in the regular release (ir) form. The generic substitute is "mixed amphetamine salts" (MAS). Several companies produce it, and the generic is much cheaper than name brand Adderall.
Currently there is no generic option for Adderall XR...though I believe one is slated to be produced starting this spring ('09). I believe that Vyvanse is less expensive than XR at this point...probably because the maker, Shire, wants to steer Adderall users onto Vyvanse before XR goes generic.
anecdotally: MAS (ir) kicks in for me in 1/2 hour, starts leaving 4 hours after I take it. Adderall XR fires up a little slower and gentler for me than the ir, and starts leaving after 6 hours.