A lot of the ground systems are in place and works, but should not be considered production ready. Integration with commerce is very sketchy at best, since commerce doesn't officially support off site payment gateway, which is what PayEx is. Under the right circumstances it's possible to get it to work, but you need a deep understanding of commerce and need to modify it to make everything work.
Tagged a stable release for Drupal 6. The code has not changed since 9. mar. 2011, so we figured it was about time. Contains a few bugfixes since 6.x-1.0-beta2.