WordPress.com is awesome. Everybody loves to use this multiuser application. The only way to use is by getting an invitation from your friend. But, if you do want to blog using WordPress MU, just try BlogNeo.com. It doesn’t require invitations; just sign up and you’re ready to start blogging.
I haven’t really tried it extensively, but it seems to be pretty much like WordPress 1.5.2, but with some really nice AJAX-based interface improvements. There are still some problems for now, such as the small library of themes available, and the fact that a multi-user hosted site means you can’t tap into the wealth of WordPress plugins. This problem is not only faced by BlogNeo.com, but also other service provider using WP MU. Even WordPress.com has limitation of themes and plugins. As far as I know, it is related to security issues.