I've only been in a psych ER once, thank God, and the experience was a lot more traumatizing than the panic attack (worst in my life) itself.
To compound the badness of the situation, I went to the nearest ER, and I was living in Brooklyn at the time-- in a nice neighborhood, but 1 mile from serious ghetto territory. So the hospital was in the ghetto. Bad idea.
The waiting room was about 10 by 10, with six chairs and twice as many patients. Even if you were self-admitted, like me-- actually, I didn't even want to go, but a girl I was dating thought I should-- you couldn't leave. It was 6:00 pm on a Monday and there was one doctor on staff. By the way, Monday afternoons in the spring are the busiest time in the busiest season for psychiatric ERs, so there's literally no excuse for there having only been one doctor on staff.
The TV was extremely loud-- probably 90 decibels-- and some of the shows that played were rather violent. Since 2/3 of the patients were obviously there for anxiety-related problems, this was clearly a bad choice.
You had to ask the nurses to use the bathroom, and if you pissed them off, they'd threaten to bump you to the bottom of the list. Since I was "compliant" and got out in 7 hours, I can't imagine how long the waits are for the people who were uncontrollable.