I really enjoyed the previous Terminator movies, which is probably why
John Connor is a bit player in this movie. The real hero is actor Sam Worthington, who plays a Terminator (Marcus Wright) with a human brain and heart.
The movie feels like someone's fan fiction, because it rewrites canon from the previous films whenever it wants to insert a hero bigger than the savior of Humanity. Heck, this Terminator even gets a girlfriend.
There's plenty of action in this movie, so if that's all you want, fine, go see it. But if you're looking for good characterization, a plot that makes sense, or any semblance of a good ending (the last 15 minutes get especially bad), don't see this movie.