Ask HN:What e-commerce package should I use?
SwellJoe: Magento is a more modern application, and looks significantly cleaner internally than Zen or osCommerce.  They also have significant forward momentum--it seems to offer the right solution set to a large class of users, and so it's rise in popularity has been dramatic (I get yelled at daily because our product can only install it on a few very modern distros, because of the PHP 5.2 requirement).I don't get a headache (much) looking at the Magento source, but it is a huge project with hundreds of files, and I can imagine it would take a long time to grasp it when it comes to customization.  But, the other two you mention are also subject to this problem, and their code is a bit more old-fashioned (and given that the PHP community has only recently begun to pay attention to maintainability and readable/testable code, that's an important consideration).  Though none of them seem to have any unit tests, so I guess it's still not quite caught on.