If that describes you, Tumblr suggests that you download an update. There is a link on the Tumblr announcement page that you can click to download the update.
In addition to the update, Tumblr also suggests that you change your password if you have been using either the iPad or iPhone apps to access Tumblr. The reason for the update (and the suggestion that you change your password) is in response to a security issue. Something in the iPad and iPhone Tumblr apps allowed passwords to be compromised “in certain circumstances”.
It appears that if you only access Tumblr from their website that you won’t need to download the update. It also sounds like people who used the Android app to access Tumblr are unaffected by this security glitch.
I just started using Tumblr fairly recently, but I realize it has been around for quite a while. The biggest benefit I’m seeing so far is that I can “follow” the Tumblr accounts of my friends directly through Tumblr. For me, this means I can take their Tumblr accounts out of the Reader I’m using (FeedWizard). It’s much easier just to check them out through Tumblr itself.