I'm testing out GSON and have tried some others.
GSON does an excellent job of serializing (converting object to json) complex objects with basically no changes or thought at all on your part, but is a bit slow and memory intensive.
The GSON Roadmap website indicates they expect version 2.0 to address some performance issues and that it will be out in Oct 2011 (ending soon). So I'm hoping they deliver that because I really need it.
Other libraries (sorry can't recall names right now) don't seem to serialize as well. Some may only look at public variables in your classes or just call on public methods that look like getters/setters. Gson doesn't do it that way, and will grab everything.
I haven't done much yet on the deserializing side (converting JSON back to Java objects).
