As background: the TransformXml MSBuild task is typically invoked in a custom build step to mark up a web.config file with per-environment configuration; its flexible directives offer highly granular control over the insertion, removal, substitution and transformation of existing configuration hierarchies.
For those using the TransformXML task (typically in a Web Deployment Project) I raised an issue against Visual Studio 2010, in which the file handle on the input file was not released, meaning that following transformation the source file remained locked. As a result, the best way to transform a file was first to rename it, transform it, and then copy it back, leaving the "locked" file to be freed up later.
I just heard today that this has now been resolved in Visual Studio 2012 RTM. That's good news, because Web Config Transformations offer a lot. An intelligent, automated build process will swap in the relevant transform(s), making it much easier to synthesise the Developer and Build server builds. This makes for a simpler and more exemplary build process, and with the tighter coupling comes a correspondingly quicker response to Developmental change.
Oh, and don't forget -- it isn't just web.configs you can transform. You can transform app.configs, or indeed any XML file that honours the task's schema and hierarchical rules.