From a business perspective, I find the dual license model they use pretty interesting.
The author, Peter Rodgers, also has very thoughtful answers in the associated discussion thread. My favorite:
As a general observation. Our feeling is that with the XML-service world, the application must eventually handle the XML message no matter what the protocol. For many XML processing problems we’ve found that the later you bind to procedural objects the better - ie when you have the choice, keep your system loosly coupled.
Amen to that.