We have tested JIRA, ZenDesk, Desk, Kayako and even Redmine - and none really gave us what we needed, a flexible yet extremely easy-to-use support system - handling hundreds of support tasks every day by a handful of experienced support staff.
Does Atlassian plan to develop and release a "JIRA Support system" any time soon? JIRA is to extensive, with dozens of fields here and there, and - well, its too complicated for agile support staff as JIRA is more directed towards development issues.