Not really, but Tumblr has a lot of features to make re-sharing other people's content easier. - Mark Trapp

i see it as a more immediate way of posting something, tumblr for shorter posts, a blog for something more extensive. Like tumblr guys say, a blog is a journal, tumblr is a scrapbook. - Oscar Juarez

Nah, they just make some things easier than others. Like screwdrivers with different handles (not even heads) - Roberto Baca

I used to think the same way, but since I discovered Posterous I have changed my mind: I use it to post to my main blog and to my Tumblr, and lately I'm starting to consider to make it my only blog. It's so easy to use! - Jordi Soler from IM