Is it fair to say that this man had as much of an impact on modern or popular fantasy as Tolkien?
Never thought of it like that, but he probably has. (Although, most of what he's done springs from Tolkien tropes, which are themselves lifted from earlier Norse and Celtic mythology, so... not too fussed.)
Without him, I doubt that Don Delillo and Phillip Roth would have gotten as far. Great man.