Is this plugin good to use on a large site with many products or should I look for another solution? I am concerned with the security as well the ability to scale. Anyone have any experience dealing with this plugin for sites with many products?
WP e-Commerce is probably the best you are going to get while still keeping within the confines of WordPress. Keep in mind WordPress was designed originally for blogging, but it has expanded to do so much more.
For a really big store it is probably a good idea to go to dedicated store software like Magento or OS Commerce. If you want to start out with WP e-Commerce it should not be all that hard to move to another store with all your products once you outgrow WP e-Commerce.
I came across this question that may be of interest to you:
Actually WP e-Commerce can handle as many products as WordPress can handle Blog posts. In other words thousands and thousands of products :)