Tumblr is one of the fastest growing social networks online, so the release of an official client on Microsoft's mobile platform was a welcome one, and points to a maturing software development platform. Given Tumblr's audience skews towards the younger generation, I'd expect a lot of the pay as you go handsets, such as the Lumia 520, to have this app installed rather quickly.
It's also nice to see an update so soon after the first release. This isn't a fly by night checkbox client, Tumblr is putting some effort into the app. I'm expecting to see more updates because of this... and because of that I want to use the client more. It's a nice positive feedback loop.
You can pick up the updated client direct from the Windows Store.