I have been writing the odd post over on my Tumblr blog. It's a really graceful and user friendly tool. However, it's not the best if you are hoping to really jazz up your blog with widgets, etc. There's little space for them.
Also, Tumblr has a strong policy of endorsing pure, expressive blogging as opposed to using a blog to make money. Here's a post by someone who had his account suspended.
Here are some tips on how to avoid this outcome and keep your Tumblr account.
UPDATE: It's not just marketers who should be careful with this blogging platform. They have been known to disable blogs that have been built solely for the purpose of making fun of other bloggers.