I have to say that WPEC is close to being a great plugin. However, where is falls short is in its execution of what it's supposed to do. Upon installation I've had problems with variations (not changing the image or price), shipping (flat rate showing $0.00 for shipping in a region left blank) and payments (Checkout loops back to the cart rather than processing the payment using PayPal Payments Pro).
Now I realize that this is a free plugin and, as plugins go, it's fairly complex. However, it's the lack of support for these issues that acerbates the problem. After spending some time on the GetShopped support forum, I found there are other individuals who were having the same issues I was having. Some of these posts had been up for months without any type of response or attempt to resolve the issue.
I had thought about upgrading my payment gateway to thier Gold Cart plugin. However, I've found that this plugin also has its problems and support for it is almost non-existent.
If they fix the bugs and make a concerted attempt to support this plugin I may change my mind. Until that time, this is the last time I install WPEC.