I use tumblr as a outlet for the pictures I love (it's a "tumble" or a mixture of quotes, photos, and thoughts.) i wouldn't really use it as a blogging format. But to each their own!
I always thought it was for pictures, but our FRG and ombudsmen use it as a means of updating and it annoys the Jesus out of me, because it's hard to subscribe to... and unless I can subscribe to something, I guarantee I won't think to look at it!
Tumblr is more a thing if you want to post, pictures, quotes, chats..almost like twitter I'd say. For blogging I LOVE the blogger here and I'll probably never use my tumblr for blogging. I have a link there to my blog here and use tumblr for quick posts, quotes, pictures.... check it out:http://www.silkecruz.tumblr.com