Depending on how big they are, they're fucking intolerable. It feels like a red-hot blade made of pinheads spearing every nerve in your flank and no position you get in can alleviate the pain. The largest I ever had was a 6mm, which is the borderline between passable and impassable.
The good news is that codeine or basic opiates will take the worst of the pain off and make you functional. The trick is getting that first dose in you without puking it up; I have a mortar and pestle to grind up a codeine pill for insufflution, to get past the blood-brain barrier faster, then I can take the pills afterward because the first dose kills the nausea. I always keep a supply of hydrocodone on hand in case it happens again to keep me out of a $5,000 ER bill.
The trip down the ureter is the worst. Once it gets to the bladder, it's tolerable as it makes its way out unless it's a spiky motherfucker.