Context: I worked at a university lab and was funded by PSI, so I didn't have too much contact with PSI.
All communications with PSI seemed professional and well-coordinated. I enjoyed the work I was doing and the lab I was working in. My advisor was extremely friendly and supportive.
What I wish was different
The timesheet situation was kind of weird because my research project required less work than 40 hours some weeks and more work than 40 hours other weeks, but since I could not log more than 40 hours on my timesheet I could not get paid for working extra hours some weeks.
Advice
I feel like most of the advice I could offer would be specific to the lab that I worked in, but just in general, as a research assistant make sure you have an idea of the broad scope of your project and what you want to achieve so that you can be checking your progress.
