I loved the film the first time I watched it, and I've watched it a couple times since. Overall, I really don't think it's that far off the mark. In real life hackers have shut down power grids, nuclear plants, banking networks .. .. and on and on. Anything that is computer controlled (and these days there is a lot more) has a potential to be hacked. Systems doesn't even have to be directly connected to the internet, resourceful (and in the case of the nuclear plant, let's be fair, most likely state sponsored) hackers will find a way.
To top it all off, damn did I have a crush on Jolie. In fact, I'm quite tempted to dig it out and have another watch today.