Expression Manager is now functional in the limesurvey_dev_tms branch.
This wiki page (http://docs.limesurvey.org/Expression+Manager) is meant to be the definitive reference for it's capabilities, syntax, etc. It currently contains enough to give everyone a flavor of ExpressionManager's capabilities. As time permits, I'll add more examples, references to test scripts, etc. I also plan to host a working copy of this branch so that users can play with pre-built surveys that show ExpressionManager's capabilities (at least until this functionality is baked into the CI branch).