I received one of the new Pro-Tools docks to demo at the end of June. After a week of mixing with it in my system I'm still on the fence about keeping it - but not for any feature/quality reason. (Edit July 26, 2016: I have kept the dock and love it - definitely speeds up my workflow significantly and allows me to spend less time looking at the computer screen.)
There are many videos online already about the dock and it's features so I'll keep this quite brief, and I'm not going to talk about the iPad app as currently the app is exactly the same whether or not you have a dock. If you have an iPad you should use the app regardless.
1) automation panel - this is an obvious huge benefit to anyone mixing with either the S3 or a set of artist mixes. If you're used to mixing on a large format desk this will make you happy.
2) wheel - now I didn't think I'd be that excited about the wheel but I have been using it for things I never found comfortable with other wheels - primarily horizontal and vertical zoom. In fact I leave it on horizontal zoom most of the time and find it so much better than zoom in/out toggles or presets.
touch strips... More assignable buttons are great, but I don't get these touch strips - the tactile feedback is not great (I find I have to look at them) and their sensitivity seems unreliable. I'm probably missing something cool, but I have no idea what it is.
physical size - imo the unit is too deep and puts the iPad further away than I would like - to a point where I have to "reach" for it. I find myself not doing things on the iPad (that I had been doing before adding the dock) because of the physical distance. As of writing this I have not come up with a placement that I am satisfied with.
Overall I think it's a great combination with either artist mixes, or the S3 - and a big step-up from the iPad app alone. Still having put a lot of time into finding a good ergonomic set-up I am hesitant.