I have tested most of the "helpdesk"/"ticket" extensions on JED and have been a loyal user of RS Tickets for a couple of years, but on a project I needed one key function that RS couldn't give.
Allowing "employees" from the same "company" to be able to share "tickets" submitted by their colleagues.
Helpdesk did it!!
This extension is huge yet easy to use once you read the docs and understand how you wish your helpdask to work.
There a lot of features, dynamic tickets, Departments, Custom Fields, Mail Notification, FAQs, knowledge base, as so much more.
Allow your "staff" to create tickets for your customers, assign the ticket to other staff, automatically change status based on the previous state, add attachments, view ticket logs, create new users, this list keeps going on an on for the feature that are available on the front end.
Set specific departments for users to add tickets to, even create a staff department that only staff can view. Just blown away by how powerful this is.
As for the extensions support, it works, they use this extension themselves, the developers respond timely and are proactive regarding feature requests and any minor issues I have found.
Do not be put of by the price, what you get in comparison to other solutions is a well designed, smart looking, feature packed professional helpdesk solution.
I have not been as excited by an extension since I started using K2 all them years ago.