Please check the links in the project home page on how to get started with Horn. It’s very simple and actually works as promised. One thing you need to remember is that you will have to install a subversion client and a Git client for Horn to check out the source code from the projects you want to build.
I had two issues in my Win7 machine, where I tested. The first issue is related to Git really and not to Horn and/or Win7. Make sure that you have the path to your git.cmd into the PATH of your computer.
The other issue seems to be a concurrency issue when trying to read a Temp file. Not sure how this files is been created just yet. This happens while trying to build complex projects like MvcContrib. Re-issuing the install command will “resume” the build. It took me three attempts to build MvcContrib.
Even with this issue (that may not be a Horn issue at all) I think that this is a very important project for the .Net community. It makes building this complex projects with all the dependencies a routine task. You don’t have to spend hours tracking down the project, the source code and pray that everything builds and works well together.