Existing AA and UA premium seats are no difference but the legroom. No change in service that I could tell. On some AA types, the MCE seats happen to be in a quasi-private cabin simply because of where the bulkheads are located. Our ORD-DUS flight last summer was like this...I think a 767. It was a nice, quiet seat with just two rows in the cabin.

Delta appears to throw in upgraded food and free drinks. I've never flown it though. Nor do I know if they tend to price it higher than AA MCE or UA E+.

I received an email recently about AA starting to roll out a true Premium Economy product in 2016. Don't recall what markets/types will get it first.

A good Premium Economy ticket *can* get expensive. I've seen them get closer to J than Y on occasion. The basic extra-legroom seats on U.S. carriers are just a fixed add-on to whatever the going coach fare is.