No real side effects aside from ever so slightly heavier periods in the first couple months after insertion
It was overall very good. The onoy side effdcts i experienced were slight weight gain and i got pretty bad acne while i was on it which i had never experienced before. I ended up going on accutane for a year which cleared my skin, however the acne came back exactly the same once i stopped. I had a minor issue where my string went missing (after an ultrasound it was clear the mirena was still correctly positioned and working). The missing strings made removal really painful as my doc needed to fish around a bit to get it out, but the pain was only during the procedure and with pain killers probably would have been minimal. Overall I stopped using the mirena because my acne was terrible (like 25+ spots at any given time) and I didn't want to go in accutane again.