I've been using RT for more than one year. It is way better than Bugzilla. Some pros that come to my mind:

• Very flexiable ACLs. You can configure easily who can see, reply, comment and admin tickets and ticket queues.
• Unlike Bugzilla most functionality is configurable via web interface.
• Unlike Bugzilla it is designed to be extandable via add-on modules.
• It is written in much better Perl. I'm sorry, probably Mozilla developers are good C++ developers but their Perl is crap (at least it was crap year ago when I looked on it last time).

Not sure how RT compares to PerlDesk as I have never used it but I urge you to look on RT too. It is very good.

Update: I forgot about another pro: RT has two-way email gateway. Unlike Bugzilla you don't just get notifications for new posts and comments but you can reply on them using your faivorite email client.

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Ilya Martynov (http://martynov.org/)