I spent some time 'kicking the tires'. Version 4 looks to be a major improvement both in edit/build/test mode and in web app development functionality. It is great that a good IDE like NetBeans is available for free. However, since I earn most of my living designing and writing Java code, I feel that it is worth while buying a 'best of breed' commercial product (which, in my opinion, is IntelliJ).
Same thing with Common Lisp: there are awesomely good free Common Lisp compilers available (e.g., OpenMCL, CLisp, SBCL, CMU). Still, the free tools are just not at the same level as the Xanalys LispWorks and the Franz's Allegro CL. The commercial products provide a richer development environment and library support. Allegro CL is out of my budget price range, so I happily use LispWorks.
Python is the other programming language that I use a lot. I am still trying to find a 'best of breed' Python IDE. I mostly develop on Mac OS X so the ActiveState IDE is not a possibility. I have tried a trial version of the WingSoft IDE (runs under X11 on Mac OS X) and it is good but just not at the same level as IntelliJ or LispWorks. So, I just use Emacs (for about 15 years, I did all of my coding in Emacs, but productivity-wise, Emacs is simply not competitive with commercial IDEs like IntelliJ).
Anyway: great work on the new NetBeans release!