Why should I use IHttpActionResult instead of HttpResponseMessage?

You might decide not to use IHttpActionResult because your existing code builds a HttpResponseMessage that doesn’t fit one of the canned responses. You can however adapt HttpResponseMessage to IHttpActionResult using the canned response of ResponseMessage. It took me a while to figure this out, so I wanted to post it showing that you don’t necesarily have to choose one or the other:

public IHttpActionResult SomeAction()
{
   IHttpActionResult response;
   //we want a 303 with the ability to set location
   HttpResponseMessage responseMsg = new HttpResponseMessage(HttpStatusCode.RedirectMethod);
   responseMsg.Headers.Location = new Uri("http://customLocation.blah");
   response = ResponseMessage(responseMsg);
   return response;
}

Note, ResponseMessage is a method of the base class ApiController that your controller should inherit from.