It's pretty intuitive. However you're basically screwed if you need to change some functionality. The programming is all procedural and the template files are inundated with includes, and requires, in place of the core template system. Rather than using objects, parameters are set and globalized, making it impossible - exempting a global site search - to change some of the core functionality.
And the back breaker was the in-ability to set fixed shipping cost's per item, or the ability to create an extension due to the lack of OO use.