osCommerce was a pain for me, and looks like the project is very outdated.
After working with CakePHP 1.2 and 1.3, I am convinced using the MVC approach is much cleaner and organized, and quicker to develop. It's so much easier to figure out how the application is structured.
Pinnacle Cart has some great features but seeing a lot of database queries mixed in with html and other functions just makes me wish an application like that was better organized, it would increase productivity.
Also, why aren't the popular applications like WordPress, Joomla!, etc. using the MVC approach?