I was nervous. It was actually easier than i thought and less scary than it looks on telly. They lowered the lights, which made me feel less self conscious, and it didn't hurt or pinch whatsoever, just a bit of squeezing pressure no worse than if you laid on your front. The "plates" they put your boob between were perspex, not cold steel as i had somehow envisioned.
Hope this gives some comfort to anyone nervously anticipating theirs!