I upgraded to IntelliJ 4 a few days ago. Actually, I always considered IntelliJ 3 to be just about perfect, but version 4 has a slightly nicer UI and many small improvements.
I am now using the Tomcat integration for JSP/struts development. When I start a Tomcat 5 instance in a command shell with my web app, it takes about 20 seconds to start up. When I start the same Tomcat instance in IntelliJ, it starts up in 6 seconds - I am guessing that this speedup comes both from not having to start a new JVM, and running Tomcat in embedded mode.
Anyway, combined with a symbolic debugger for JSPs, the faster code/run cycle is great for my general work process. Most of my work involves either web applications or web services, so small improvements add up.
I like Eclipse and NetBeans, but I think that IntelliJ beats them, hands down: I find myself much more productive using IntelliJ. For a few years, I used Borland's JBuilder (Borland gave me a free Enterprise JBuilder license because I had written so many Java books). I have not used JBuilder in a long while - it might be comparable to IntelliJ.