A quick note on support and some of the haters (gonna hate!).
1) I'm not employed by Instinct, the company behind the plugin. I don't personally provide "support" for the plugin.
2) I am an author on the plugin - I'm the only author of it that isn't employed by Instinct. I spend a lot of time, unpaid, making this plugin better.
So, some support education -
1) There's a lot of great e-commerce options available. If you look at the free ones, none of them provide stellar support. It's a hard thing to provide support for a small free plugin - try providing support for a free plugin that has been downloaded 2.1 million times. Even the strictly paid plugins (Cart66, Shopp, etc.) leave quite a bit to be desired in the support realm.
2) The support Instinct does provide has gotten better in the last year or so. They've been working on it. Does it still suck sometimes? Sure. I have yet to work with any company that has a 100% track record.
3) Someone mentioned all the 1-star ratings. Quick thought on that - WPeC was the very first plugin around (by several years) that handled general e-commerce. The first version was released for WP 1.5. Take five minutes and go install 1.5. See what it looked like. If you're a developer, take a few minutes and figure out what hooks were available to use, let alone custom post types and taxonomies. Then imagine building a user base of hundreds of thousands, maintaining backwards compatibility for them, and still innovating and at least keeping up with the newer options - if not out-innovating them.
Yeah...that's hard. It's really, really hard. There's bumps along the way and people, rather than realizing it's a small company trying to do the best they can - that ultimately, it's real people behind the software - people like to leave crappy little notes and one-star ratings.
I was almost one of those people. But back in 2010, this was all there was, and it was buggy - like, really buggy. But instead of complaining (to be fair, I complained a bit) - I emailed Dan (Instinct CEO) and said "Hey, let me help you.". Open-source software is rad because of the possibility of collaboration - not complaining.
So if you think it sucks - great. Patches welcome. Be part of something bigger than you.