The tool has proven to be flexible and highly useful. I've used Zephyr and another forgettable tool in the past, and so far have found Testuff to provide a very nice experience in project and test case management, and test assignment/execution. I have not explored the requirements feature, nor have I made much use of the reporting feature. However, and above all, the customer support I have received from Testuff to date has been OUTSTANDING. For this reason I think this is a great solution that you can be confident about trying.
Note that my project team sizes are small and generally last only 12 weeks, so I have no idea how it works with larger products with thousands of test cases and tens of testers.