There's nothing they can compare it to...I think I read somewhere that the human bodies 3 worst soft spots are the eyes, the...male parts (unless you're http://www.youtube.com/watch?v=a86cQobU-n4 :unsure: ) and adam's apple (which it does, even a small hit makes you gasp for breath).
Way off, I'm afraid. According to http://www.ehow.com/way_5444485_body-part-least-sensitive-pain.html: "The body parts most sensitive to pain are the lips, face, hands and fingers, feet and toes, and tongue."
That's because they have the densest concentration of pain-sensing neurons.
Though I don't want to undermine the hurt of getting hit in the junk, it simply doesn't compare. I've taken a fastball pitch there, without a can, and it's not even in my top 5 painful experiences. I'll volunteer for that a hundred times over before volunteering for childbirth once. For me, the most painful was the aftermath of getting my 3rd front tooth removed, because the dentist had to drill away at the cement covering it without anesthetic. Power tools on raw gums... Not fun!
Oh, and I've also whacked myself very hard in the left eye and am nearly blind in it as a result. It went numb instantly - didn't hurt a bit.
Ripping my ALC in half, then trying to stand after doing it. I took two steps and then my leg bent the wrong way...

Did that skiing last year. Painful yes (and worse than being hit in the junk), but mostly it just felt... Weird! Before I even landed, I heard that *pop* and knew it wasn't gonna be good.