The reason why I've worked so much with John is that I think his style in precisely in keeping with the aesthetics of the early (77-82ish) modules released for 1st Edition. Just take a look at the following 5 illustrations and you'll see what I mean. Not only is John a great artist, he's also a good guy to work with for another important reason: he's always on time. I think over the decade that I've worked with him he's been late (never more than a week) on something like two or three times, which is astoundingly consistent delivery for this industry. If you're looking for an artist, I can't recommend him enough.
Oh, did I mention that after working with John for a few years he told me that his wife Daisy can do color work with him? Now most of the cover illustrations in the Advanced Adventure line are done by the duo. I've attached five of the covers as well so you can see that together they really create some great art.