I did it last week. I got a reply back, although a nasty and hypocritical one to say the least. My supervisor acted like she was brand new. She knew EXACTLY what I was talking about, yet still insisted on playing dumb.
I am just waiting for my pink slip, because she is not willing to stand up for us any more. We are truly on our own. They change the rules every day, and treat us like sweat shop workers in Taiwan (please no offense to anyone). In a nutshell, she didn't care.
PS: I hope your supervisor is the same one as mine.
PPS: I've been here for 20 years and I am going out like THIS?! The horrible anxiety I have, but I am still glad I wrote the letter.