Someone really had to start this fan war 
No seriously, I have been using JGrasp (www.jgrasp.org) since I first started teaching myself Java and I loved it. But I decided that it was time to move on to more professional utils.
(I know Caspian will be all over Eclipse ;D )
Here we goā¦
They are both good. Try them and see which one you like more.
Iāve used both Eclipse and NetBeans extensively in professional settings, and while I have a slight preference for NetBeans, as IronclawsBt said, theyāre both excellent tools and the choice between them comes down to personal preference. Iāve never used IntelliJ but have heard great things about it as well.
On the other hand, Iāve never heard of jGRASP until now, and, having taken a look at it, think that, while perhaps ugly, it does have some cool features that other IDEs could benefit from.
I canāt tell about NetBeans, cause Iāve never used it . But what Iāve noticed is that youāll become a fan of the first one you start using (since both are good) .
So pick your choice carefully !
It was actually the opposite for me. I used Eclipse first then switched to NBs. I first used Eclipse a few years ago and at the time the interface was kind of clunky. Since then, they have made a lot of improvements. Now it is pretty much entirely personal preference. If there is some specific feature you really want, there may be some difference, but for the general user there really is no wrong choice.
I think Eclipse is more popular among the enterprise java programmers (which accounts for most java developers). IBM created the Eclipse platform back around 2001 to compete with NetBeans, and for the most part they have won.
Having said that, I have been using NetBeans since 2001 ā I was also one of the original user of Eclipse. I ultimately stayed with NetBeans because I find it less confusing to navigate. I still use Eclipse now since many of the google dev tools (android, gwt) has plugins for Eclipse, so you may want to take that into consideration.
I tried NetBeans a while back (and IntelliJ IDEA) but nothing ever comes close to Eclipse in terms of sheer functionality and ease of use. Eclipse can be a bit cranky and buggy at times. Itās caused me to lose a substantial amount of hair. And always off the top, never above my arse at the back. Grr.
Cas 
You must be kidding!? I used all three and while I donāt argue about all the other aspects of them, prefering Eclipse over others because of itās ease of use is beyond my understanding :o
Why does it seem that both are equal?!? Aghhh. Seems like the only way to determine which one I like is to create a game in both.
However, is it me or does Eclipse looks prettier? ;D
Changing the Look and Feel to Nimbus makes it better looking ;D
What features does it have that others donāt?
what kind of functionality? Usually you have to install third party plugins to be able to do basic things with eclipse like maven, mercurial, subversion, profiling, bugtracking etc. What eclipse distribution do you use?
What are maven, mercurial, and subversion? 
just a few examples of some common developer tools/methods. maven is a build system which is esp. good in dependency management of third party libs (another one would be ant), subversion, git, mercurial are versioning systems. Wikipedia will give you a better description if you want to know more.
At work I have Eclipse and Netbeans open all day every day. Netbeans for the Netbeans RCP app development and Eclipse for the EJB backend. IMO, the Eclipse editor (the actual text editing component) really blows. Itās ugly, buggy, and slow to edit large files (10,000 lines+). Netbeans on the other hand fails to start a managed instance of JBoss because it canāt handle the 25MB log file in itās console output window (ie this means we canāt debug into J2EE apps in Netbeans probably could by attaching the NB debugger to a running JBOss but no one has arsed about with that).
What it boils down to for a newbie is: in Netbeans everything just works after one download. Eclipse you have to chase plugins down and itās more work to set up. Otherwise, they provide the same functionality. The point above about plugins for Google goodies is a good one. Netbeans generally has plugins for Google projects like gwt and appengine, but theyāre not as mature.
Edit: Another thing is gui builder: Netbeans has long had a very good gui builder in Matisse, Eclipse had no real free option here. Google bought the best commercial Eclipse gui builder plugin and made it free. So now Netbeans gui builder supports only Swing, while Eclipse with Google plugin supports Swing, SWT, and GWT.
I have converted many of my friends that were using eclipse, to intellij. I have never heard of any beeing converted the other way. I have used eclipse for almost two years at work (and I donāt look back). But I have to admit I have also met fanatic eclipse users that didnāt convert to intellij.
IMO, Eclipse is the best free ide but Intellij is the best.
Donāt miss that intellij have a free version (called community version) without lots of plugins for instance spring-support. It you donāt want to do enterprise programming, you are probably going to be ok.
I used to be an Eclipse user but switched to NB some years ago. I still prefer NB but use Eclipse when I need to (for a plugin that I canāt get on NB). Iāve used IdeaJ, but donāt prefer it to either NB or Eclipse. Theyāre all good tool thou. I also still use emacs for some tasks (never Java coding thou)
Well, the āControl Structure Diagramsā (those little graphics on the side of the code) - they look nice - and the viewers (that show a graphical representation of a data structure) - they look cool, too. These canāt be found in Eclipse or NetBeans.
Emacs??? you are insane!
vi is the way to go!
I still chuckle to myself inwardly every time I type āviā into launchy to start Visual Studio. Sad, I know.
I do use vim occasionally to make small changes to files on a webserver (even though pspad is more comfortable for that really since I never really learnt how to use vim very effectively).
Are you on drugs??? Nothing NOTHING NOTHING completes with emacs. And this is from someone who canāt stand Richard Stallman. I tell myself that Guy Steele had to been the more important contributor. (Jokingā¦well, kinda.)
Why not use them in parallel ?
At the beginning its a little bit of effort,
but once you learned the layout of a project of the two IDEs on your disk
you can use the power of both easily.
Thatās already very much of the game.
If you have some more ācourageā get into the āantā tool and you are probably
almost a java (developer tools) Pro !