I'm trying to get rid of some stuff that's been cluttering my basement, so I used a preview version of the site to set up my own store, rafestore.myshopify.com. It took all of 10 minutes to get started, get my PayPal and credit card information into the site, and post three items for sale. The site's visual templates are pretty good, and if you have HTML skills, you can create your own as well.
My experiment isn't the ideal case for Shopify, however. It's really meant to be a persistent online storefront for an ongoing business. As such, it has almost everything a small retailer will need and nothing it doesn't. It handles credit card and PayPal processing for you, and it lets you easily tag items and create both static and dynamic catalog pages. For example, a static page might be "manager's picks," in which you drag the items you want into the category; a dynamic page could be "items under $15," which automatically pulls items in that meet the criterion. A great example of a real Shopify store is Reorganize.ca.
Shopify isn't free. It charges you a percentage of your sales income. But it does not charge a hosting or setup fee (as giant hosted commerce site Yahoo does, for example), so if you sell nothing, you pay nothing.
Missing from the site are things you might find in other e-commerce solutions, such as coupon features and marketing tools. Also missing is the ability to offer customized (for example, monogrammed) products or to sell downloadable items. These and other features may be added soon, either by Shopify or by users, once Shopify opens access to its site via an API (it's in the works).
At launch, Shopify will be clean, simple, and easy to use. Since it has such a nice interface for managing an online storefront, I expect that the founders will be barraged with feature requests, mostly to allow the export of Shopify catalogs to other online venues, such as eBay, Froogle, and MySpace.