Zen-Cart is an excellent shopping cart/ecommerce solution due to the numerous administration functions it provides. It allows you to create multiple looks that you can easily switch between by picking from the template list. I like its product options capabilities that allow me to offer different colors, sizes, configurations, etc. and display an option picture and description next to a radio button, checkbox, or dropdown list. I also like its multiple image capabilities that automatically look for additional images based on the file name without having to populate the database with a filename for every image.
Zen-Cart has advanced capabilities beyond other open source scripts and the Zen team already has plans for new versions that other cart developers have never considered implementing. Zen-Cart evolves much faster than some of the other open source projects that I have looked into. Its developers thrive on a community effort and by helping others out.
I starting using Zen-Cart back in October 2004 and sure it had some bugs, but not any more than scripts that I have paid for. Sure there were so many options that it seemed almost insurmountable to be able set up the functionality that I wanted, but I know I can't just click on one button and expect to know how to do everything by brain osmosis.
Overall Zen-Cart is definitely the premier open source ecommerce solution, but it�s not for those that want something turn-key while someone holds your hand. You can easily run into problems installing it and configuring it if you have never installed or configured a PHP application on a webserver before or you are the type that �just does it� without taking the time to prepare or read instructions.
I give it a 4 for functionality and usability and an extra point for being open source for a grand total rating of 5.