I've never had the job of doing research on Help Desk products, but I've used Atlassian Service Desk as an agent, dev support, and as a customer and it was decent experience all around. Of course, it's most useful when you're using Jira.
We believe that JIRA Service Desk is the best for most use cases, and we're committed to aggressive pricing.
Let me know if I can help out at all.
Scott Farquhar, CEO Atlassian