If you've seen the trailers, where key plot lines were revealed, you already know that in this movie, when Kyle Reese is sent back to protect Sarah Conner, she already knows he's coming and already has her own friendly terminator, played by, who else, Arnold. He saved her when she was 9-years-old.
This movie kind of mashes up events in the first two movies and the TV show, pretty effectively, I'd say. You've got your original Terminator played by a buff CG Arnold, who doesn't stand a chance against old-Arnold ("I'm old but not obsolete"). You've got you T-1000, made to look a lot like Robert Patrick from T-2, for whom they've set an elaborate trap.
There are some problems with the time travel concept. Old-Arnold seems to know too much about what future Skynet plans to do. Reese, while in transit to the past, somehow picks up memories of his childhood self born into the alternative timeline created by the attack on 9-year-old Sarah. This is a very key plot point without which the second half of the movie couldn't exist. But, hey, it's time travel, it's complicated. Just remember this quote from Old Joe from the "Looper" movie:
"I don't want to talk about time travel because if we start talking about
it then we're going to be here all day talking about it, making
diagrams with straws."
There are some wonderful action set pieces, some of them which will remind you of scenes from the first two movies. There are a few set pieces which make no sense, like the one where the school bus that Sarah Conner is madly driving across the Golden Gate Bridge flips high into the air, does a double somersault, lands on it's roof, slides through a number of cars, and over the edge where it hangs. All without Conner and Reese getting a scratch.
My wife and I enjoyed it but my daughter was somewhat dissatisfied. She's not very familiar with the first 2 movies which I think hurt her fuller enjoyment of the movie.