I went through the experience myself - and result was a series of articles on my blog:
- http://virgo47.wordpress.com/2011/01/30/eclipse-vs-intellij-idea/
- http://virgo47.wordpress.com/2011/02/22/from-intellij-idea-to-eclipse-2/
- http://virgo47.wordpress.com/2011/03/24/from-intellij-idea-to-eclipse-3/
- http://virgo47.wordpress.com/2011/02/28/why-to-synchronize-with-svn-in-eclipse/
- http://virgo47.wordpress.com/2011/04/10/from-intellij-idea-to-eclipse-4/
- http://virgo47.wordpress.com/2011/06/28/from-intellij-idea-to-eclipse-5/
They are all biased towards Idea (obviously) but full of relevant Idea-Eclipse comparisons, keyboard shortcuts, little stories and observations. In generall, they are both very good IDEs, but if IDEA is 100%, Eclipse is 90%, maybe even 95% - with biggest these differences:
- different mindset is needed, Projects, Modules, Workspace may mean different things or are not used at all in one or the other IDE, you have/need Perspectives in Eclipse, not in IDEA, etc...
- quality of default Maven/SVN support is better in IDEA (it is also built-in), much smoother and less problems + 3way diff in IDEA is just great, generally Eclipse guys are so scared of merging - and now I understand why
- IDEA is far more polished, less graphics glitches and much better default colours for highlighting, etc.
- free version of IDEA does NOT have so many things as you can have with Eclipse with all possible free plugins - Eclipse plugin ecosystem is just so big!
- IDEA is just way smarter in margin cases when it comes to completion, refactorings, and these other little things where IDEA was the top of the class the whole time since 2000
I was lucky I didn't have to convert in the end because we use Maven projects that work just fine in both IDEs. However I still use Eclipse for other projects (xtext).