I've logged in and there are some UI problems (i.e. the html puts the mailbox listing below the change locale line, and i created a few errors will browsing around the site). Also, i needed to log into my account with email@example.com and this automatically created a "from" address of firstname.lastname@example.org@domain.com (becuase of the way the server was set up. I would recommend adding adding checkbox in the admin UI that says whether or not to append a domain name to a username at login time. Likewise there are some bugs on the create folder page. BUT I know how development goes, and i may be telling you something you already know.
I for one will be VERY happy to have such a package to offer my collegues webmail on our site. I am going to develop a vpopmail admin user interface package this fall. This too could be integrated well into a webmail client... its simple, it will allow for the creation and removal of accounts/forwards and autorespond messages (that are automatically removed when the time comes, etc.). One thing i would encourage you to do is to look at fastmail.fm for user interface suggestions. I know that that's their full time job, but as far as UI's for webmail go i think they are by far the best. I love how i can select a message and file it in any of the folders on the left for example, and this could be integrated into your package.