For the past three years I've been using the Jikes compiler to build GNU Classpath, but unfortunately Jikes development isn't very active anymore. I wanted to start playing with the GNU Classpath generics branch (the branch that contains the classes that require 1.5 specific language features), so I needed a compiler that could compile the generics branch and was freely available. The Eclipse Java Compiler was the obvious choice and while I ran into two bugs when I tried to compile the generics branch, the bugs were fixed within hours after I filed them, that kind of responsiveness is confidence inspiring.
The next IKVM release (hopefully due next week) will still be built with Jikes, but future releases will most likely be built with the Eclipse Java Compiler. To make this easier I've used ikvmc to compile it to ecj.exe and created a Windows installer that installs ecj.exe and adds IKVM.GNU.Classpath.dll and IKVM.Runtime.dll to the Global Assembly Cache. If you're not on Windows, you can download ecj.exe in this zipfile. Note that ecj.exe requires Mono 184.108.40.206 or later.