We're self-hosted using Mephisto. Why? I like the self-hosted solution because it feels more professional. And I have more control over theme and plugins (it looks exactly like the rest of our site). I went with Mephisto for simplicity (in interface, not installation).

Though, I'm starting to wish I had gone with something more traditional for the framework, like Wordpress. It has way more support for plugins than Mephisto.