My reaction to the booster was worse the previous 2 shots combined.
I had Moderna for all 3. The first 2 there was the normal arm soreness after a flu shot. No big deal. I had a bit of dizziness the morning after the 2nd shot. That was about it.
I had the normal air soreness the morning after the booster. It gradually worked its way to pain, and I couldn't move my arm in certain ways. Couldn't raise it fully extended above my shoulder. It was really pretty bad (although catching myself wandering around the house with a T-rex arm was slightly & morbidly amusing). Still sore 48-72 hours after the booster. I also chills for a few hours that started up in the middle of the day. I have a bit of drafty office window, and I kind of assumed that that was it at first, but that wasn't really the culprit.
But, hey, those short term side effects were known & understood and I'll take those over the possibility of being strapped to a ventilator or developing myocardia or those types of things.