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.
Wrap-Up: Commerce Server 2007 Architecture Series
Well, as you might expect, Commerce Server 2009 is just on the horizon and a CTP has been released in December.
Announcement: Commerce Server 2009 & December CTP Availability
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.