I am using the WPMUL Support system plugin and it is working out well but there are two key features I feel it is missing.
1. The ability to edit tickets - this is essential.
2. Email piping so users can view and reply to messages from within their email client. This will remove the need to visit a web page to view/reply to messages. My users expect to be able to do this.
I really hope you take these suggestions on board and add them in the next release.