The first half of the movie has some bearing on reality. You can see the underlying structure of how they set up the plot really is based on current technology and the flaws therein.
It is a very scary and serious movie when looked at that way. Fortunately, the movie does not take itself seriously. And the second half of the movie is so implausible with huge plot holes and physical impossibilities that you just hang on for the rollercoaster and have fun.
There are a few times when the age of John McClain becomes an issue. He looks pained and worried in scenes in this movie that he never would have in the previous ones. But that is a minor quibble and actually just adds to the character development.
FYI - its hard to sit and watch Justin Long (especially towards the middle of the movie when he is talking to other hackers) without thinking "Hi, I'm a mac." Sorry, I had to. :)
Anyway, its a good summer blockbuster movie, losts of fun with at least a marginal grounding in reality (sort of). :)