It is not an exaggeration to say that this plugin has cost our company several thousands of dollars in lost revenue due to trouble shooting and getting clients off of WP e-Commerce. This plugin has gone through periods of relative stability with periods of numerous bugs. For the past 6 months it has gone to another low point. The plugin support, or companies making the plugins for WP e-Commerce are terrible. We have tried 2 different FedEx plugins and 2 USPS shipping plugins. None of them worked without severe bugs.
I wouldn't recommend this plugin to anyone unless it's on a personal site that doesn't require dependability.