I agree that Magento is a properly designed enterprise commerce package and that most other OS commerce packages are spaghetti. But Magento is just... Impenetrable. You wander around forever just trying to figure out how to change some simple thing. Knock the PHP hack carts all you want, but I bet I can figure out how to graft on a quick feature about a billion times faster in that than in Magento. And when you're talking OS solutions, a great many users will be of the hacker type just wanting to git 'r done.
Magento is just waaaaaay to "pure" for my tastes.