Since “WordPress MU is no longer under active development as a separate product“, I don’t recommend using it. Instead I recommend using the setup described in this answer. It’s highly configurable you can choose what WordPress files you want on all your WordPress sites and what you want to store separately from each other WordPress site. With this setup all the WordPress functionality will remain working just like you’re running a regular WordPress installation. Except this one is way easier to maintain, uses less disk space and can be distributed across a whole server or cloud.
