Those who want to decide which URL routing technique to be used and in which scenario, can have look into "IIS URL Rewriting and ASP.NET Routing" posted by Ruslan Yakushev.
"Either IIS URL rewriting or ASP.NET routing can be used to implement URL manipulation scenarios for your Web application. ASP.NET routing is a solution that is optimized for ASP.NET, thus it may be preferable for Web developers who design their ASP.NET applications from the ground up and want to have a clean URL structure. IIS URL rewriting is a generic URL manipulation mechanism that addresses a multitude of scenarios. In particular, it can be used by Web developers as well as Web server/site administrators to enable clean URLs for existing Web applications without modifying the application code."