Other platforms I have evaluated: Kayako, Fogbugz, Bugzilla, Assembla, FreshDesk.
I ended up picking Zendesk for several reasons, but there was really 3 reasons why I ultimately picked them:
1) Customization - Zendesk provides powerful capabilities for making sure your support portal looks and feels like a native part of your own product. Customers dont feel like they are being pushed off to some other service in order to receive support and that is very important to me.
2) Speed - Zendesk is fast. Really fast. Not just technically, but with their product design as well. I can load up and work on tickets at blazing fast speeds because they have designed the product to make sure you succeed in your task - help customers. With great macro integration, automation, rulesets, a fast AJAX interface, you can just knock tickets out one after another.
3) Support & Education - When you sign up for Zendesk, you are getting more than a support platform. You are getting access to a support team that is there for you - technical issues, questions about proper ways to handle support, and even life advice. Ok, maybe not life advice, but they are seriously there for you with just about anything support related you need. But more than great support, you get education. Zendesk brings in experts and other Zendesk power users to host webinars, lectures, and events on top of all of their web based documentation in order to educate you on how to succeed with your support.