Well, this Johnson+Johnson vaccine supposedly has pretty mild side effects compared to others like Moderna and Pfizer (which I hear can give you flu-like symptoms as side effects), and not only is it a one-dose vaccine, but its supposedly more effective against relatively newer strains and variants like those that originated in Africa and the U.K., whereas those Moderna and Pfizer vaccines now require third shots to protect you from those.
But yeah, I was told that side effects from this vaccine include, but are not limited to pain/discomfort wherever you get the shot (arm or leg), headaches, fever, and fatigue, and could last up to 24-48 hours after injection. It also takes up to two weeks for the vaccine to take full effect in your system.
Again, that's just the Johnson+Johnson vaccine, so I don't know of all the pertinent details regarding either Moderna or Pfizer, other than they now require three shots, and have far more severe and crippling side effects.