BuddyPress.org

@jonnylons, are you looking for something that lets users write blog posts for their sites? Somebody pointed me to a few good front-end blog post plugins. If that’s what you’re looking for, say the word, and I’ll dig ’em up.

Or, now that I look more closely at the link @footybible offered, sweet. And he mentioned the front-end approach already. I have no current need of this solution myself, as I’m not running multisite, but I am definitely bookmarking that guide in case it comes up in the future….

So, sounds like it just depends on how much control your users need and how much time you want to invest in setting that up.