If you don't have plans to see this movie, you can check the spoilers here and then come back. Also, in the interest of full disclosure, I saw this at a drive-in, but don't really think that impacted my opinion of it terribly one way or another.
I wouldn't say this was an outright disaster, but at best a bland movie with some fairly deep flaws. A really good action flick should have a clear endgame established early on and everything that occurs leading up is either a landmark or sidetrack on the road to getting there; with Terminator Salvation, you don't get the sense that there is a longterm destination, just a series of meaningless fights and chase scenes cobbled together without an idea of the big picture. Some of those action sequences are actually pretty well done and make for decently entertaining moments, but in the long run, I just didn't care. Christian Bale is one of the few action stars who can transcend just sounding cool reciting tough guy dialogue and actually make you believe in the realism of an out there premise through his intensity, but he's not in enough of this to turn it around. It also doesn't help that he gets some really awful lines. Sam Worthington is a find and will be a credible star on his own who can carry better movies than this; he's got charisma and some of Bale's intensity clearly rubbed off, though he does need to figure out what accent he's supposed to be doing and stick with it. Anton Yelchin's wannabe tough guy act as young Kyle Reese wore thin real fast and his put-on gravel voice was grating. Moon Bloodgood has a great name, but her character's actions were borderline inexplicable and I wanted Linda Hamilton to slap her in the face for setting the tough chick role back a good decade or so. I thought Common was playing a robot because he has yet to master speaking like a human being. Bryce Dallas Howard was...well, she was in this movie, that's about it. And of course I'd be remiss if I didn't say that putting the big game-changing twist in every preview and commercial certainly didn't do this sucker any favors. I will give credit that Salvation wasn't inaccesible for somebody like me who has only a cursory knowledge of the Terminator mythology, but understanding something and being entertained by it are two different animals.