The senior developer who's productivity is really important to him
The ambitious junior developer who wants to become a senior developer as soon as possible.
The people who just code because it pays well... work 8 hours per day.. and never work on projects outside of work... well... they generally end up using a free IDE.
IDEA v/s Eclipse/Netbeans
IDEA is simply more intuitive - the developers believe in "Development By Intention", so you should be able to write code that you can refactor, manipulate, or work with, even while it's broken.. and then let the IDE fill in all the necessary details afterwards. So you get the ideas out of your brain and into the code as fast as possible, and THEN let the IDE go back and handle all the mundane stuff.
IDEA is keyboard-centric - something that is very valuable for senior developers who know what they want to do with the code. It's simply faster to write code when you don't have to switch over to use the mouse again and again. Neal Ford explains this well here: http://www.nofluffjuststuff.com/media.jsp?mediaId=60
IDEA v/s Eclipse/Netbeans (cont)
IDEA was the IDE that first introduced refactoring to the Java industry, based on suggestions from Martin Fowler, back in 2001. It's editor has consistently led the top 3 IDE's as the most consistent, and most intelligent. Last year, NetBeans publicly declared that they were trying to catch up to IDEA's editor - even though they've been trying to do that for years, without announcing it. Since 80% of your coding time is spent working with just your editor and your brain, using an advanced editor just makes sense.
IDEA has the strongest cross-language and cross-technology support - our industry-leading set of refactorings, code analysis, and code assistance features work consistently, regardless of the language or technology that you want to use. When we want to cover a new technology/language, we work hard to make sure that our level of support is consistent and first class... right out of the box, without the need to search for, find, test, install, and try out new plugins... plugins that may or may not recognize each other or work together.