Drupal 6 is absolutely unsupported by the rest of the community so far, so it's a nice 'playground' more than anything worth actually developing a site on. Until the majority of Drupal 5 modules work with 6, Drupal 5 will continue to be 'the way to go' for the vast majority of users.
Unfortunately I didn't figure this out until well after beginning to experiment with 6, but once we got rid of that and went back to 5, Drupal has been a HUGE improvement over the nightmare that zencart and open-commerce are.
Still not perfect, but a billion times better.
I have a theory that it's impossible to prove anything, but I can't prove it.