There might be valid reasons to switch for sure but I like it's uptime, it uses static html and how I can login with my Google account.
IMO if you want a hosted solution go with Tumblr, if you want a self-hosted solution go with WordPress (www.wordpress.org, not .com).
You can customise a Tumblr blog quite a lot and you have the added benefit of not needing to worry about bandwidth (should you strike it lucky and land on the first page of Digg). For greater customisation though (i.e. do basically whatever you want) you should go with WordPress (by far the most popular blogging solution on the Internet).
Even after 100 times, I found it counter-intuitive and irritating that clicking "New Post" on my site would launch my email client instead of their editor. The latter is buried somewhere in Settings.
Then when I posted it, it grouped all the pictures together.
We're changing our "post" links to all go to the web editor. While email is still the #1 way our users post to posterous, those already on the web want to stay on the web.
For images: we default to grouping them together into a gallery. But you can also separate them out into individual photos.
-Sachin CEO, Posterous
1 thing i am dying to see in posterous is "preview" option plus the pages & link section is a littile clumsy.