Cerberus for us as well, it's great.
1. Multiple company support
2. Multiple staff support
3 Knowledgebase
4. Ticketing system with Fetch & Retrieve smart learning solutions.
5. Willingness of Cerberus staff and contributors to modify/customize or work with you to get it to be all it can be, and then some.
6. Modular...you can build your additions, or integrate other ones fairly easily.
For an example of a work in progress, see
http://helpdesknetwork.com It's built completely around the Cerberus Client Gui. It's a work in progress, just redesigned and still adding content. Most areas don't require you to sign in to see it, just the ticket submission and downloads, and the downloads aren't available yet.
Forgot to add, it's free if you only need one support email. You have nothing to lose by trying it out. You can update the license to multiple support addresses and companies if you need to. It's well worth the price imho.