I saw it once in the theater and just didn't get it. I had seen T1, T2, and T3 and loved them, but it had been a while.
I just watched it again last night on Blu Ray after a recent Terminator marathon and really loved it. In fact, it has stayed with me most of the day. T1 is a great film for a dozen reasons, but T2 took it to the next level by humanizing the T-800. While T3 had it's problems (I think 65% of which would have been alleviated with a different actor as John Connor), I think Terminator Salvation was very much about being a war film with a soul- something it succeeded at being.
I very much liked watching the rise of John Connor and preferred the T1 Kyle flashbacks to remain just that. I think the fall of the machines and and older John Conner would be an interesting flick to see, but not before T4 and maybe even a T5.
T1 was about a machine who, although looking like a human, had no semblance of humanity. It was very much about the difference between humans and the machines.
T2 made us realize that in Terminator world, it was possible for machine to feel and perhaps the human soul was not exclusive to flesh and bones, and was more about the capacity to care.
T3 lacked similar concepts but the nuclear blast happening was a necessary plot point in the Terminator saga and I think it's forgivable just so that can take place in the time line. I do love the idea that the T-800 ultimately kills John Connor. It is hard to believe Connor's connection with the T-800 wouldn't shape his future actions in the future in some way (and as T3 explains is his undoing).
Terminator Salvation was not a chase film like T1 and T2, but it did return to the question of humanity. Connor realizes the war isn't just about surviving, its about winning by staying true to what matters. And in this case, we see a machine who is perhaps more human than Conner himself for most of the film. Echoing T2, we see a machine sacrificing himself for the greater good.