There is a set of excellent blog posts that will tell you most of what you need to know if you are considering Commerce Server 2007.
Well, as you might expect, Commerce Server 2009 is just on the horizon and a CTP has been released in December.
One of the CS2009 features that I'm planning to check out is SharePoint Commerce Services. Here are a few blurbs from the announcement:
"Commerce Server 2009 delivers the ability to increase your business reach by making it possible to sell via multiple channels using an out-of-the-box shopping site, SharePoint Commerce Services.
...The new out-of-the-box shopping site leverages SharePoint Commerce Services, which provides a gallery of ASP.NET 3.5 Web Parts, a comprehensive e-commerce shopping feature-set, and technology integration between Commerce Server and SharePoint technologies.
...Complete out-of-the-box e-commerce shopping site in SharePoint with new search functionality, new shopping features, and what-you-see-is-what-you-get (WYSIWYG) content management and design experiences. This helps to facilitate rapid assembly and maintenance of e-commerce Web sites by business users and creative professionals."
This sounds pretty cool and fills an e-commerce gap SharePoint has today that several of my customers want.