Yes they hurt a bit and can make a welt, depending on the range. I'd compare it to getting snapped with a rubber band. When I occasionally play I wear thermal long johns (underneath, that is) and that keeps the bruising down.
Once, playing indoors, this buffed guy got shot on the neck by one of his 'friends' from a range of about 1-2 feet. So-called friend realized his mistake and let out a (nervous, I hope) laugh. Amazing restraint on the part of the victim was all that saved this guy from the hospital. I was hoping he'd get a little more than the ass-chewing he received.
My understanding is that for the colder climates, they use balls a slightly different paint formulation because even without freezing they get worse on the receiving end, not to mention other problems cited above.
With the infrequent play I've had (maybe 20 days so far) I have yet to see a fight, but I haven't played a tournament & hear they get much more aggresssive, more cheating & yelling so I 'spose it can happen.
[red][size=4]P.R.K.