I love it - it gives you enough features but also automates some of the mundane things. It allows custom templates, uploads, and just about anything you'd want for free.
For me, a blog is about the content more than anything. Different platforms don't matter to me as a reader or a person who tries to write stuff. That being said, Posterous is the easiest to work with at the moment. All I have to do is write - there is very little effort I need to put into managing it.
Some issues: If you don't like emailing posts, the WYSIWYG interface they provide isn't amazing (I like it, but it can feel slightly clunky at times). If you need widgets or a drag & drop interface for adding external content, you won't feel at home. These don't really affect me, but everyone's different.
- Tumblr is by far the easiest, and has the most features (domain mapping, CSS/HTML editing) for free
- WordPress.com is the most powerful, and has more advanced features (full site with just pages, shortcodes) than most other free hosted platforms.
Whatever you choose, make sure to use your own custom domain. Doing so, you can later change your blogging platform wherever you want.