I think of it as halfway between Twitter and LJ: you want to put up pictures and sometimes say stuff in more that 140 characters, but you don't want to commit to a whole post where you'd have to answer comments too! Frankly I think I'm too old for Tumblr myself and I'm not using it to its full potential -- for me it's just a simple and fast way to keep track of things I might want to look at again, heh.