We are running 2015.
To me it's not about being worth it but being on top of the latest technology as well as support.
If you ever have issue with your software and you contact support, they will tell you install latest version and try.
Nobody will support old versions of software and I am not talking about Heidelberg only but every other developer out there.
We have service contract and we get all upgrades/updates for free so why not use it
There was interface change, I believe you either have old UI or combo where you can switch from old to new UI.
In 2015 it is only new UI, no more old.
There are some neat improvements, nothing earth shattering but useful, e.g. if you keep your Signa JDF's in same folder and you go to import it into cockpit, as long as your JDF is named the same as your job, it will be preselected automatically for you.