Another option to consider is the e-shop plugin - not as "fully functional" as wp e-co0mmerce, but I've been able to put together some test designs that I think look as good or better than wp e-commerce.
As far as wp e-commerce being the most feature rich plugin, that really depends on what you consider feature rich. When people's shops are going off line for days or more at a time due to non-existent support, and a new, undocumented set of problems that accompany every single upgrade...during those times, WP e-commerce is hands down the most feature-LESS shopping cart on the entire internet.
This plugin has potential, but the developers' sole reliance on a guy named Shane to provide all documentation and support, coupled with their annoyingly cheery dispositions in their rare forum posts, makes the whole operation smell like its not completely above board.
Despite statements that there is no reason not to use this plugin, there are in fact SEVERAL reasons not to use it, and there will continue to be more, as there is a slow-growing body of competitive alternatives that provide better support and a lower risk of lost revenue due to implementation problems.