The best one I know of and use is a plugin at bit.ly - It works with Paypal and pretty much has everything from shopping cart option, shipping options, buy button option, you name it. And it can be configured for digital products or services. What sold me was it's connectivity with email autoresponders so that when someone buys they auto sign up to your selected email list for follow up. I've been using it for about two months now, without glitch. Great support by the way, and that is another reason I went for it - here's the link again - bit.ly
To answer your question - "Is it worth it" - well that depends on the individual and if they expect to make sales on their wordpress site - It was worth it for me because it has cut down the development time to get a shopping cart into my wordpress website, it's very easy to change settings on my sales pages, and I'm getting sales so that proves to me the process behind the plugin is working.

You just need to look at their rating to know how good they are. WP e-Commerce is recommended for start-ups company. If you want something that worths, then you may want to use this Shopp plugin. It's simple yet powerful plugin.