Neal Adams' influence cannot be underestimated. Many of the greatest comics artists that started out in the 70s and 80s, could be said to have begun working as Neal Adams imitators. George Pérez, Gene Day, John Byrne, Bill Sienkiewicz or Alan Davis all owe an enormous debt to Adams. Even today, the artists that pencil in a more "realistic" style are riding Adams' coat tails.
It's also astounding the immense gulf in quality between his 60s work and that of his peers. It's an interesting exercise to compare X-Men #55 by Werner Roth, a competent artist in the 60s, with X-men #56 by Neal Adams, both from 1969.