you know when the doctor says "You might feel a slight sting", and then proceeds to stab you repeatedly with the speed and frenzied glee of a homocidal sewing machine?
Something like that.
Actually, it depends on where you're it, and how "into the game" you are. Getting hit on the hands or forearms (the most common areas) can really sting and raise welts. Getting hit on a shoulder or a leg doesn't really have much impact, although it can raise a welt and bruise. But, you may be so focussed that you don't realize you're hit. Sometimes, as AmmoTech said, it can break skin. Excellent training aid. It can also vary on the range. If he's far enough away, the impact is minimal. Up close and personal, and you find yourself screeching like a school-girl at an 'N Sync concert. (Or maybe that's just me. I'm kinda delicate.)