I developed a problem with my sciatic nerve back in the spring of 2004 ... at least I'm pretty sure that's what it was ... burning agonizing pain running from a point in my lower back, down my left cheek, and down the back of the thigh.
I had been riding a gel saddle, and for a while (a year) it seemed fine, but that spring I had no end of trouble every time I rode that saddle. I switched to a Brooks B-17 and haven't had a problem since. I suspect the gel was putting pressure where there shouldn't have been pressure.