Skip Chapter 6 if the game's dragging on you. The new stuff is not worth it. Just go through the story content, skip the side-chapters, and move on to Chapter 7.
Chapter 5 has some interesting new unique parts (revisit Commander Goodman's house for the sidequest there for one of the best). But you can skip it if you want as well.
Don't skip anything in Chapters 7 or 8, though. The content is just way too awesome to miss.