At 30, I had treatment for cervical cancer, and during the scans I had, the Doctors told me that I had scar tissue wrapped around my uterus and bowels from 2 c sections. Which explained why I had horrid painful flooding periods, the worst of which was when I stood up in my dads house and gushed all over his antique rug in the living room.... he wanted to call an ambulance and was dreadfully upset that this could be normal.
So the Gynae consultant recommended a Mirena to me - it was a new thing that they were trialling and had good reports about reducing periods. I was so desperate I'd have tried anything. This was nearly 20 years ago now, and I've had them replaced every 5 years and not had any issues at all. Periods stopped within 3 months, and I stopped being housebound for 3 days a month. Had no side effects whatsoever, and can honestly say that for me it was life changing.
However our local services have now changed and instead of having it replaced at our local sexual health clinic (they now only treat under 25s) I had to have my latest one done at my GPs surgery. She bent one coil so I had to go back, and ever since insertion, I've had the worst imaginable pains with it that I've never had with others. Like a sharp stabbing pain that literally takes your breath away. I've got a horrid feeling this one has embedded deeply into my lining or even gone through and I'm putting off going to have it looked at.........
So honestly, they can be a huge benefit but only when correctly fitted. And sadly our GP doesn't do enough of them to quantify much of a level of expertise. If you get one fitted, insist on going somewhere where the staff are really experienced with it.