I've used both. I have a personally bought copy of IntelliJ, though I'm using Eclipse now.

For the most part, they're both comparable. Both have powerful refactoring and code-generation abilities.

For core Java development, there's no need to leave Eclipse. IntelliJ might provide some nice features, but overall it doesn't warrant purchasing a license.

When combined with web development, other languages, or non-core development, is where it easily pays off to switch to IntelliJ.

We were doing some Grails work, and Eclipe's support (via STS) was shady at best. We put off trying IntelliJ since it meant learning a new environment, new shortcuts, etc, but after about a day of playing with it, we were sold, and never looked back.

My current employer (and myself, too) had a history of Eclipse already when I started. Another dev got hired that was an IntelliJ nut. He was able to set up a working environment without "contaminating" our projects. Occasionally we would pair with him driving, and I was always impressed by the speed and quality of the IDE. Once he left I never bothered to look further.