Definitively I adopted Netbeans 6.0 for all my projects. It is so mature that the only criticism nowadays is about its startup delay. Yes. It is true, but only in the Windows platform. If you execute Netbeans on Linux or Solaris you will see how fast it is. But in all platforms it is still slower than Eclipse to startup.
The good news is that the version 6.1 promises to startup 40% faster! This improvement is very important because people say they don't use Netbeans for performance reasons, which is a negative impact on the startup process, since the use of the IDE is very comfortable and productive.
Some programmers are totally against IDEs. VI and Emacs lovers just do not accept to wait for something processing which is not their own programs. There is a controversy here: do they loose more time having some performance delays or doing everything by hand? James Gosling, Java's father, has a message for them: "Just stop!". James has authority to say that because he was responsible for the first version of Emacs on Unix. He said that Emacs doesn't have significant evolutions since the seventies and he does suggest to adopt an IDE definitively.
Well, make your choice and work with an IDE. Ignore all those who say that it's better to learn Java using notepad. It is not true. The IDE really transmits a lot of important concepts which is not visible on ordinary text editors. I do suggest Netbeans because I love the community around it, but Eclipse is also an excellent choice.