Develop with pleasure. That's the IntelliJ IDEA slogan, and at least through Version 5 I think they nailed it on the head.
So this product review is simple: If you're a Java developer go get an eval copy of IntelliJ IDEA right now and give it an honest eval. You won't be sorry.
I do think IntelliJ is the best Java IDE going right now. It combines great technology with speed, and a clean, efficient user interface. My biggest technological concern is that Version 6 seemed very bloated when I tried it. It loaded up very slowly and seemed to use a lot of memory. From what I saw a lot of that slowness came from new "teamwork" features, and since I'm not on a team right now, I just use Version 5, and I'm very happy with it.
That being said, if the IntelliJ people have a problem -- besides the bloating issue -- it's that Eclipse has come a long way, and of course Eclipse is free. As I've written before it's hard to compete against a decent, free IDE that does almost everything you do (and possibly more), with a large plug-in community supporting them.
More on IntelliJ versus Eclipse
Comparing Eclipse and IntelliJ, a friend said it best: Eclipse is like using DOS, and IntelliJ is like using the Mac. You can probably get the same things done in Eclipse, but it's a little more work, and it's not nearly as smooth.
Free versus smooth
So I think you have to think about it: If you're spending 6+ hours working with a Java IDE every day you have to decide if you want to work with one that's free and well-suported (Eclipse) or one that's not free, and IMHO, is a better product (IntelliJ). My personal experience is that the best craftsmen I've met prefer to work with the best tools, and again IMHO, IntelliJ is the best IDE going.