I am using both IDEs (although Eclipse is really a 'platform') and it's hard to decide which one is better for my development work. In my experience, more people use Eclipse than use NetBeans. Most projects I've worked on lately have used an Eclipse base. The beauty of Eclipse is in how 'pluggable' and extensible it is. There's just a lot of plugins available for Eclipse - and very useful ones at that! People I've talked to say it's easier to develop plugins for Eclipse than for NetBeans.
The extensibility of Eclipse though may cause first-time users to find it tedious. The basic distribution of the platform does basic things. If you want to do web development, for example, you'd need to separately download JEM, EMF, and other dependencies to get it to work. Because of how detached this process is, the folks at Eclipse also have a version of Eclipse (WTP) that comes with all dependencies preconfigured.
In NetBeans, on the other hand, you only do one download of their web pack, and you got all you need. The preview version (6.0-M) is slow to start (up to 2 minutes starting up??), but works great. All the tools you need are well integrated in the base version. I'd say NetBeans is more straight-forward and user-friendly than Eclipse. A lot of useful features have been added to NetBeans IDE, though others have been removed (and why?). For example, in 5.5, you can refactor and choose access modifiers for the class attributes on one screen. In 6.o, you only do refactoring with no options to change access modifiers. So you must manually add "private" where necessary. Little inconvenience.
The other drawback to NetBeans is that it only supports SJSAS for Java EE development. Otherwise, you can also have JBoss and Tomcat for J2EE 1.4. Eclipse offers a wider range of options for servers, if you specify .server properties correctly.
It'd appear that Eclipse is more advanced, but I think NetBeans has gotten better too. The stigma that NetBeans is a Sun tool is slowly eroding, paving the way for wider acceptance. I'd recommend that a developer be able to use both IDEs - although you can essentially do the same tasks with either.
My strategy: I use NetBeans to do samples and prototypes when I work on an Eclipse-based project - and then move the ideas to Eclipse for production, and vice versa. Keeps me plugged in on both IDEs.