I like NB because of the extendability - lots of people develop modules for the NB Platform (you can also easily create your own). For example, the Sun Java Studio Creator is just NB with some added modules.
As IBM Websphere Application Developer is an extended version of Eclipse. The drawback on WSAD is: It is not pure Java, there are some DLL files bundled. So far I had no luck to get it running on Mac OS X