He is probably the most talented guitarist they've ever had but during the time he was in the band guitar solos weren't "popular" on records so he never got to show his true skills on an Anthrax album. I'd say John's playing on FAK is some of the best solos on an Anthrax record.
He's definitely a fast shredder but he reminds me of Chris Broderick - I can't really recall a single note from his solos on FAK. Unlike some Dan Spitz' leads or Rob Caggiano's on WM (these are my favourite solos). Saw Paul Crook live twice and I'd say he had the best tone of them all.