Meh.
Minor Spoilers
Well the film is better than T3 but it still doesn't even come close to touching the first two. The story is kinda forgettable and in the end pretty pointless. Marcus, while the best character in the movie, just feels thrown in with no real reason or purpose and in the end he's just a completely wasted character. It's like they threw time travel in just for the sake of just having it. I think the most annoying thing in the film to me was Elfman's score. It's the whole Roger Rabbit "Shave and a Haircut" syndrome. The Terminator theme is in there but only 3 notes of it, which he plays over and over and over and... without actually concluding it. Made it feel like a made for tv movie. I did like that the film still had a bit of that road trip movie feel that the previous films had, and thought the T800 battle at the end was ok (CG Arnold wasn't nearly as bad as what I was expecting).
Hollow, pointless, and forgettable. Not good, not bad, just meh.