Funny ... I had a nurse who had 3 kids tell me she'd rather give birth again.
When they start, the pain is actually in your lower back area. Imagine someone hitting you repeatedly with a baseball bat in that area and you'll come close to what it feels like. The majority of the pain is actually caused by the stone moving through the ureter to the bladder.
The first one I had was jagged, and got stuck at the entrance to my bladder. Was in and out of the ER for four days (gotta love morphine though). Had to be surgically removed, and I'll spare the squeemish from exactly how it came out, but I'm sure you can guess.
Second one hurt just as bad, but passed on it's own after a few days.
Here's a trick that will help ... in addition to whatever else (like Percoset) they give you, ask for a 800mg prescription of Ibuprophen. My urologist told me it's fast acting to the kidneys. Had that for my second bout and it made it almost bearable. Also, if you are prone calcium stones, the doc will likely tell you to have something acidic in your drink (like lemon) or drink cranberry juice. Guess it helps keep them from forming. So far it's working for me, and I knock on wood every day.