I built the open source IDEA 9.0 git snapshot - works fine
With the Apache 2.0 license, it will be interesting to see how it is used. It is a large git clone, but built easily using ant.
I get a free commercial license for IntelliJ IDEA (as I used to get free Enterprise JBuilder licenses from Borland) but I still plan on following the open source IDEA project - hopefully interesting things will happen! I use Eclipse a lot just because the Java AppEngine support is so very good, but for plain old Java coding, I like IDEA.
The open source edition of IDEA is great for plain old Java coding, BTW, but is missing JSP + Tomcat development support (but NetBeans does a good job for J2EE-- development, and who does J2EE development anymore :-)
It takes a while to do a git clone and build the IDE (builds versions for OS X, Windows, and Linux as the default ant build target) and since the build process is so easy, it was not much fun, so you might as well just download a built version for your OS platform if you don't want to peruse the source code.