How should we judge NFL quarterback Michael Vick?

As an animal lover, I was appalled when the NFL allowed Michael Vick to play pro football again after his dog-fighting episodes. But now that he's doing well, part of me wants to cheer for him, telling myself that he's a "reformed man who deserves a second chance". Is that rational of me? How do we know if someone has truly turned over a new leaf morally after prior bad acts?