Well, I've had lead bolts for a while now, with the default values from the raws, and i can report the following field experiences:
1) Squad of lead-shooting marksdwarves will consistenly beat their wood or silver counterparts in a totally unarmoured stand-off. Cumulative damage is the key here - the lead bolts rarely cut the skin, but it's bruise, chip, fracture every time. And a head-shot is an instakill. It's close though, in 10v10 there will be two or three survivors (usually badly injured) but always on the lead side.
2) In fortress mode, it's great ammo for hunters, because their prey has just bare skin. Because it causes more pain than the wooden or silver bolts, the chance of a retaliating prey getting a lucky strike are much much lower. If you've got lots of lead, make lead bolts and give it to the hunters.
3) Much like silver, it's pretty useless against metal armoured opponents. However, it could be very good for crowd control, since the non-cutting subdual action makes it like a modern rubber bullet. I haven't had the situation come up, but a guardsdwarf with lead bolts would take out a berserker with ease, and without spraying blood all over your nice clean dining room.
4) It's cool, because it's a significantly different type of weapon. Like hammer at a distance.
So not a replacement for iron, but if you're short on the good stuff, it's not completely useless.