I just did five years in Federal prison and did two stretches in the SHU (basically solitary), totaling about two months. First time was for drawing on a paper food service hat. Second time was for being a smartass to the prison shrink.
Me, I didn't mind it so much. Peace and quiet (though occasionally you get a screamer on the range). Got some reading done. Meditated.
But you only get to make one call every thirty days. No coffee, no commissary. The cops keep the place cold like a meat locker. Lights never go off.
It's not for violent criminals. You get sucker punched or stomped and you go to the SHU for 30 days for an "investigation". You file a grievence against a staff member and you go in for a 90-day "investigation". You get the flu or scabies and you're in there for two weeks: quarantine.
The really violent people end up on a USP or AD-Max in Florence, CO.
I didn't mind the SHU because I enjoy a bit of solitude now and then. But in California, there are guys who've spent decades in the hole. That totally fucks you up.