I thought Terminator Salvation was okay as an action movie, the writing was poor though.
I thought the actors did a good job with the sub-par screenplay.
Bale was fine. I doubt he was in cruise control, he's a method actor, you could even notice him talking and acting similar to John Connor in interviews for The Dark Knight because he was in the middle of filming TS.
The writing was the problem with that character (and the other characters, but to a lesser extent). John Connor wasn't even a leading role in the original screenplay; Bale was originally asked to play Marcus (which is why Marcus gets as much if not more screentime than JC). They were doing re-writes on set to get him into more scenes and it shows, the character has no depth.