Since this series has long been and gone, and several other reviewers have described it perfectly, I'll just make a few brief points. Anyone who has ever spent time working in the environment this series is set in will appreciate just how hard it can be to distinguish between Doctors and Patients. Psychos works because it captures the very essence of the subject. At the same time it allows both the story line and characters to develop in a way that pushes the very edges of the boundaries of reality. Are units like this staffed by people like this? No. Does the program portray the essential nature of all those involved? Absolutely.
It's worth pointing out that the series is now available on DVD.