I should clarify a couple of things here. First, it's the machines that manage to justify their insane behaviour, by having an actual plan wherein their actions make sense; the humans really are behaving like pillocks. Second, the machines' plan is hopelessly complicated and error-prone.
I was going to describe the plot, but it seems pointless to do so. Instead, I'll skip straight to the acting. Christian Bale growls his way through, making you wonder whether he should have been playing a Terminator instead. It's left to Sam Worthington to carry the film, and he does just that. It's his movie, all the way through. Think Max Beesley in Survivors. Moon Bloodgood makes a good fist of Worthington's love interest, and gets to play an active character along the way. Anton Yelchin - Chekov in the new Star Trek - is okay as Kyle Reese, I suppose, but not dynamic enough.
The action sequences are all rather good, of course, but it's not really a worthy successor to the original. In fact, I don't think it comes up to the level of T2.