It is only recently I realised you can use Windows environment variables in your msbuild scripts. If you want to allow overrides of build settings on a per-user basis but don’t want force anyone to create a file just to prevent the build from breaking, you can use something like this:
<Import Project="$(USERPROFILE)\Customisation.props" Condition="Exists('$(USERPROFILE)\Customisation.props')" />
This shows to neat things. First, the USERPROFILE environment variable is referenced using the exact same syntax as normal MsBuild properties.
Second, the inclusion is optional thanks to the Condition on the Task testing whether the file exists and only allowing the import if it does.
You can read more details about this feature on MSDN.