We mainly use Polarion ALM for requirements engineering as well as task planning and task execution, but we mainly use it as a documentation and traceability tool.
I work in the medical device development field. Even though Polarion is quite expensive, it's quite a good solution for medical device development in general, especially for software development.
Within our previous company, not that many people used Polarion as it was only used within our department relating to medical device development. I'd say around 20 to 30 people were using it. It's hard to say as we had around six licenses with some floating licenses.