RailKing Subway cars are very nice. I have a set of R-17's and they look good, as do the other types I have seen. As subway cars don't have much applied detail, the RailKing cars are fine. Depending on which car and which era you are modelling, you may want to add small details such as radio antennas or safety chains. There's a lot of video and photos out there both of stock and modified cars to reference.
There is a lot more "scale" RailKing now.
For example, their RS-1 is to scale, as are the subway cars.
The short locos are seen mostly in starter sets.
The details are also improving. Again, the RS-1 as an example. The latest run has lighted number boards and a lot more paint detail. MTH improves them as they re-release them.
There are scale length RailKing diesels.
This video shows a lashup of a RailKing and a Premier.
http://www.youtube.com/watch?v=k5cwgn6_wMwI've has a chance to compare a Dash-8 Premier and a Dash-8 RailKing side by side. The differences were in the details. The RailKing had cast on details, the Premier had separate parts and more intricate detail painting. Dimensionally they were the same and both sounded the same.
Yes, some RailKing has been "selectively compressed" but not all.
I'd look at the individual model before passing it up just because it's in a RailKing box.