WP E-Commerce does a nice job filling the "product store" gap left in the available WordPress plugins. Overall, it does a nice job of getting a customer through a shopping experience, then on through checkout. But, it needs much refinement.
On the plus side, when compared to an enterprise-level E-Commerce solution such as Magento, WP E-Commerce is a breeze to customize. Well, most of the time.
Once you dig in, you might find a few conflicting classes, and by that I mean, classes that WP E-Commerce tries to generalize across the site, that tend to conflict with many WordPress themes.
In the case of the shopping cart page, a class of header is applied to the shopping cart table header. This may not have been the best design decision. In my case, the theme I was using had already defined a header class, for use the template heading area.
This post is not intended to scare people away from WP E-Commerce, but rather to encourage people to work through the issues. Make sure your browser is equipped with a good set of development tools for debugging CSS.