This software has a countless amount of bugs, issues, and lack of features that will make any serious web developer pull their hair out.
1) Shipping is broke in the latest version. It just simply doesn't work. Even on a clean installation of WordPress. During the checkout process, you will simply be redirected back to the checkout page in a constant loop.
2) There is no product filtering and search filtering. Your end users will not be able to sort products by price.
3) You cannot customize the packing slip without hacking the core of the plugin.
4) Poor developer documentation. No real explanations as to what most common functions actually do.
5) I can almost guarantee you will have to purchase at least ONE other add-on/plugin for WPEC if you want to accomplish anything like special shipping and proper product importing.
This solution is NOT meant for people trying to build a serious E-Commerce website. Maybe if you're selling a couple digital downloads or a couple small products. Unfortunately for me I wasted over a year hacking this crap to work on a store with nearly 3,000 products and spent thousands on other 3rd party plugins from Visser Labs.
Do yourself a favor and if you're going to do a large E-Commerce website, you probably shouldn't use WordPress at all.