I have joined the Microblogging bandwagon and Posterous is my weapon of choice. I chose Posterous over Tumblr because the UI of Posterous is clean and intuitive and even the Posterous’s iOS App is pretty well done.
Personally, I think Tumblr has too much features for me such as the sharing of Text, Photo, Quote, Link, Chat, Audio and Video. I personally going to use a microblogging service to share photos with some text only and hence Posterous fits my needs.