Re: supersized.org down?
I believe I can speak for Jannis that if the service ever were to shut down, all of you would be able to get a SQL dump to migrate your blog otherwhere.
Since what Jannis does is free, I'm sure he can't really give any guarantees about how long it will last. But it's worked for several years now. If you really, really want to be sure about your content and data, of course you should always host the blog yourself. Every other free service could also always shutdown due to unforeseen issues, you'd never know.