film SLR has a different focal plane, length and aperture to digital SLR (ccd is smaller, set further forward and generally more sensitive)
the ring will also alter focal length and aperture
(it might even improve it!)
but at the end of the day it's a "bodged fix" .... but it will save a few hundred quid ...
jessops or similar MAY let you try before you buy ... but they are reasonably cheap .... certainly worth a go.
I haven'y used one on a digi ... but on my old clockwork - after a couple of asted rolls ... it worked quite well