Yes, Tim. It is a great product. I can say from our experience, we narrowed down some of key performance issues in our implementations with the help of PSM.
Basically we can use PSM for
1. Proactive Monitoring - Monitoring dashboards are really useful System Health, JVM, Cache Status. We do have a dashboard up in a monitor to watch our production environment.
2. SLA Management - We can setup measure, threshold on response times. This is very useful if you have SLA with your business. Common End User Operation dashboard is quite useful for checkout, checkin,create move,copy etc response time monitoring and it will report out SLA violations.
3.Diagnostic Aspect - This is the killer - We can drill down from the user action to code execution to db queries through all the server layers. Purepath drill down is very useful. If you have UEM, you can drill down from desktop browser to network to webserver to MS to db.
4. Incident Management - PSM has ITIL ready incident management capabilities, but I have not used it so can't speak to it.
You can browse through http://ptc.com/go/psm for more details. Also you can have a look in to our UEM demo video here
A screenshot from our PSM Dashboard

Thanks
Binesh Kumar
Barry Wehmiller International