If I wasn't tied up with about a dozen other social networks, I might actually use it.
Although I do have an account I have basically made mine into an archive for all my status updates by using http://ifttt.com/ . That way when I post a status on Facebook or twitter It sends it to Tumblr. But that is about the extent of it.
If I didn't have a Facebook or Twitter I might have used Tumblr.