WCF Web API with Web Activator package

In these days, I had read some blogs and a lot of post in internet about WCF Web API. And I also have curious about it.  It is not new technology. And after I watched some postcast. I recognize it is a good one that I must to researching. RESTfull is a hot topic and have a lot of argues these days. WCF Web API is built on top of WCF service. How does it good?