I have been using Twitter for a long time. It is at the top of the micro-blogging structure. I also use Jaiku and Pownce, but I think that Tumblr has something to offer, something to compete with against the rest of them. I would say Pownce comes the closes, as far as features, when compared with Tumblr.
Users can have custom themes, in addition to being able to edit the provided themes code and throw enhancements to the pages. Just like I did earlier with Disqus! Tumblr has plenty of room to grow when it comes to features to be added. But in my opinion it is the better one of all of them.
I hope that it will soon pick up speed and become more popular amongst the micro-blogging community. I did try to find a WordPress plugin that integrated Tumblr the way Alex Kings plugin integrates Twitter. But, I did not even find one that can interact with it. Helpfully soon someone will produce something.