Callum,
You asked Dmitry but I will chip in
The last I looked at Netbeans (which was actually to play with JavaFX), its support for JavaScript and CSS, and Subversion integration was no where near what IJ can do.
Those are the two concrete reasons I gave to my boss when recommending buying IJ licenses instead of using another free
IDE, but really it is the many small things that IJ gets right that make it the best
Java IDE, IMHO of course