Tumblr is... not my favourite format, really, but I must admit that it's pretty useful for art. Or for making a scrapbook of other people's art via reblogs. And I love making photosets (except for the part where tumblr sometimes decides to change the dimensions in an update and everyone, including me, throws a fit).
Not ideal for longform journaling (my favourite) but then again, it was never really intended for that. Those looking to use it for art - go ahead, it could be worthwhile for you!