WP MU is amazing! Normally, creating many WP blogs on a single domain generates significant overhead - in terms of disk space and server load. With WP MU you can probably create over 10,000 blogs on a single box!
And the fun don't end there! It's extimated most of the existing plugins work with MU. Also, a new type of plugin is supported that's automatically activated for each new blog.
You can hack the code to death, as with WP.
WP MU is really delightful! It gives automated SPAM a new meaning
