just saw this documentary and was inspired to check out his recorded work. always thought nillson was to soft/aor to garner my interest, but on more critical listening have concluded that he really was a melodic genius and arguably the best white singer of his era. someone in the film that he influenced the beatles as much as they influenced him, and that may be correct--mccartney's solo work in particualr sounds sounds very heavily indebted to him.
on a largely unrelated note, check out "last days here", a documentary about the drug-crazed lead singer of the obscure (but actually outstanding) heavy metal band pentagram. interesting and weird, with a surprisingly happy ending.