Underrated: Octavarium. This album was written to highlight every aspect of the Dream Theater sound, and James responded with a diverse and consistent performance. From the smooth and subdued melodies in The Answer Lies Within, to the Muse-esque falsetto in Panic Attack, to the clean but aggressive delivery in Never Enough, to the entire box of Crayola in the title track, James nailed the different styles required of him for this record, playing a huge role in it ranking so high for me.
Overrated: This is harder to pick, as James is very good at delivering the exact vocal performance required for a given album, and I feel really bad about judging his live performances due to his food poisoning incident, but I'll give the nod to Awake here. This isn't to say that it's bad, as I love his vocals on this album, but in general he had two modes of singing on this one: raspy mids and clean mids. He does it well, and he can't be entirely faulted for the range of the melodies on the record when there were four other guys also writing vocal parts here, but as it stands, he doesn't cover as much ground stylistically as he does on other albums.