George and I both use Zen for our wives' stores. It's a bit hard to come up with a word to describe how I feel about the product as a whole....(especially the templating system and how you have to go about customizing things)

here's a choice of two words...

good? or bad?

I maintain a couple of Zen-Cart installations for some clients and once the system is configured and setup, it runs itself. It has proved to be very stable.

Customization is a whole different matter. The organization of the code is a mess in my opinion -- though it has come a long way from it's origins of forking from osCommerce. The biggest problem with customization is not the difficulty of customizing the code (it is hard), but that it makes upgrading the software more difficult later.

The benefits: it is free, it has a very active developer community, it has a decent number of plugins and theme templates, and it gets security updates on a fairly regular basis.

Both installations used to be custom coded shops. They were then migrated to osCommerce. Then to Zen. Of the three solutions Zen is the best so far. But, that doesn't mean there aren't any better solutions out there.