Painful! The nastiest injury I've sustained was from a humble tenon saw. I had my left thumb steadying the sawblade along the marked cut, and had just begun to cut when I inadvertently withdrew the blade too far and immediately began the next forward stroke. The lower corner of the blade went through the end of my thumb about halfway down the length of the nail and a good way through it. It did smart more than a little.
It was patched up with the first aid kit, but on the way home on my motorbike the dressing began to leak, and the blood congealed inside my glove. Getting the glove off wasn't a fun experience either.