If you treat this place as a tube/train station, then..it is all fine: you have good connection with any part of the city, a lot of shops/restaurants in and around the station and the information centre is very useful and all the staff are friendly.
However, there might be a disadvantage for you that this place is always crowded and you have to be really lucky to take a seat somewhere. Plus the fact that during the major renovation process that takes place there's quite poor signage.
If you treat it as a touristic attraction (for the 9 and 3/4 platform or anything else) you'll be disappointed..There's hardly anything movie-alike in that place.
As I said before, there is a major renovation taking place and there is no such thing as the charming station that Kings Cross used to be.
The 9 and 3/4 platform is somewhere hidden in a tunnel..again, nothing interesting.
There are plenty of places in London where Harry Potter movie series was filmed except Kings' Cross..try there!