I find that there are general steady improvements everywhere. The routing seems much nicer - especially how it will try to follow around other traces to the point you specify. This makes it so much easier and quicker.
There's no reason not to upgrade and give it a try. There's nothing that has got worse.
Yes I use it a lot. There's some really nice new features in it.
The design manager in the board editor is really useful, it gives much easier ways of finding/viewing/manipulating nets/parts within the board. I used it while routing some DDR3 memory to help me do the length matching as it reports the routed lengths a little more clearly than other methods.
Again in the board editor, the MOVE command has had a major revamp and now supports preserving angles on traces and polygons when moving things around. This is so much better than the old free move in most situations but it's possible to change mode to the old one if it is required.
In the schematic editor there have been lots of changes around how busses are handled. Previously there was no support for correctly creating and managing busses so I had some ULP to help sort this but now there is direct support which makes things much more slick.
There's loads of other updates, way too many to list here off the top of my head but it is a big step up from earlier versions. One thing to note is they've changed the way they handle the settings which used to be in a common .eaglerc file hut now are split up by version. In addition they've changed the way the default resources are stored/specified to create a clear distinction between the Autodesk supplied "examples" and user owned data.
