I just got back from a midnight showing--I wasn't planning on going originally, but my brother and I were bored, so we said "Hell with it" and went. I was planning on seeing it this weekend or next week anyway.
I really liked it. It was not a cinematic masterpiece by any means, and I'd still put the first two films ahead of it, but I thought it was a big improvement over T3. The action scenes were mostly pretty cool, though it started to get a little ridiculous at the end, and the story wasn't all that bad.
My biggest gripe is actually not with the movie itself, but the marketing:
Oh, and I thought Anton Yelchin did a great job as Kyle Reese. He sounded almost exactly like Michael Biehn in some of his lines, and did a good job mimicking his mannerisms and facial expressions.