However, this didn't work for me with the following error:
Failed to generate a user instance of SQL Server due to a failure in starting the process for the user instance
Wow, that's not too helpful guys. The real problem: I had configured my SQL Express instance to logon as Network Service. User instance mode requires LocalSystem. This can be changed by in SQL Configuration Manager. (Double click on the offending instance and change the logon type to LocalSystem).