My wife got hers on Monday(she's a nurse, and had her first shot in January).
She felt pretty crummy Tuesday, but was fine other than a sore arm Wednesday. Actually two sore arms since she got the flu shot at the same time and the person giving it insisted they had to be in separate arms.
BTW, my wife mentioned to me too-since there are so many injections being given now-that there is a proper location for IM injections. What she was taught in nursing school was to find the bottom of your shoulder joint, go two fingers down, and give the shot at the bottom of there. My COVID booster and flu, which BTW were right next to each other, were given in that general area. Both of hers were way up in the shoulder, and she thinks that's why both were so sore for a couple of days. Little side note, but it seems sloppy technique is kind of common now(I had a different vaccine back in August that was given way up in my shoulder, and yes it was sore for a couple of days).
A few others-my parents had boosters at the end of last week. My dad had COVID in December, and just like his first two shots, the 3rd knocked him for a loop and made him genuinely sick(fever and the works) for a few days. He's said he'll keep getting boosters as long as they think we need them, but he's not looking forward to having any more and hopes this is the end for a while.
Last one-my grandfather-in-law, who is fully vaccinated as of March or so but hasn't had a booster, was feeling bad first of the week, went in, and tested positive on a rapid screen. They said vaccinated individuals sometimes do test positive on rapid, but he was showing symptoms so they didn't send one off. He most certainly has it-he hasn't had much more than a mild cold, but has been grumpy that nothing, not even his evening beer or glass of wine, tastes right.