IUSE='experimental' use experimental && EGIT_BRANCH='next' Calling 'use' in global scope is forbidden. And I don't think you should really let USE flags decide which branch to use. Multiple ebuild versions seem saner for this purpose.
I'm currently waiting on the uzbl team to fix a failing combination of USE flags (http://www.uzbl.org/bugs/index.php?do=details&task_id=382) before pushing a new ebuild.
uzbl is out of tree now.