If you like the experience then that's great and if you make sales even better.
I have never used Photium, what are the benefits of it?
Why is ZenPhoto so good?
From my work background, I get the chance to fully edit the code/css to how I want to vision the site. You can add categories/sitemaps etc. Also adding your own code lets you include google analytics, which lets me see which specific terms, users are coming in.
At the moment Zen Photo is at an early release, but its built from the owners of Wordpress, who have BBpress too.
Also its FREE! Rather than spending around £200 a year on Photium, I can spend that on a new lens or equipment.