Necro!
Basically, for the first five levels or so, I felt that the WAD worked well enough, especially in seizing elements from the Doom Bible and plonking them down into the new engine. However, once I hit M6 or so, it seemed to take a downturn: The ammo distribution seemed to assume that you had dug out all of the secrets and found the rifle, so there were numerous points where I was totally dry on M7 and M8, especially the latter. I don't really think a WAD should assume secret-finding.
As for the backtracking, I didn't really mind... But then again, I'm a huge HeXen fan, too.