I am currently evaluating TestRail and my experience has been almost entirely positive about the product.
The one gap is in the coverage of requirements (references) against test cases. I know that TestRail isn’t attempting to be a requirements management tool, but it would be fantastic if there were a way to import or specify a complete overall list of external references (ie. JIRA keys) for a particular milestone, and have additional reporting on the test case coverage of these references.
This way not only can I be sure that my references are tracked against test case passes and defects (as you already provide against the reference field on test cases), but also that my test cases have actually been written to cover all the references I needed for this milestone in my external system.
A small secondary request (that I see has already been requested, but please add my +1!) is that the same hover functionality available via the JIRA_REST defect integration be extended to requirements so that I can see core details when hovering over the reference ID.