By far Tumblr is the most unreliable, followed by Posterous. No surprise to me, Blogger.com is the most reliable blogging-service.
That covers my experience, as I have every single blog and podcast-feed on blogger for years. And I never had an outage. No mess with updates and customizable, including use of Javascript. So you can do what you should as a blogger: Blogging.
Source: Pingdom
