Test cases are an essential asset to evaluate software quality. The research community has provided various alternatives to help developers assessing the quality of tests, like code or mutation coverage. Despite the effort spent so far, however, little is known on how practitioners perceive unit test code quality and whether the existing metrics reflect their perception. This paper aims at addressing this gap of knowledge. We first conduct semi-structured interviews and surveys with practitioners to establish a taxonomy of relevant factors for unit test quality and collect a dataset of tests rated by developers based on their perceived quality. Then, we devise a statistical model to measure how the metrics available in literature reflect the perceived quality of test cases. The findings of our study show that readability and maintainability are the key aspects for developers to diagnose the outcome of test cases and drive debugging activities. On the contrary, code coverage metrics are necessary but not sufficient to evaluate the capability of tests. Finally, we discover that available metrics are effective in characterizing poor-quality tests, while limited in distinguishing high-quality ones.