Obviously not everybody reacts the same way to any medication, but I have some Mirena horror stories.
I got pregnant on the Mirena, and that resulted in mc#1. I didn't have any medical management because Planned Parenthood couldn't help me. I miscarried alone in my bathroom, and retained some "products of conception." I bled for 10 months straight, to the point that I wanted the damn thing out so I wasn't bleeding on my honeymoon. I was told it would be wasting money, and I should leave it in longer to see if the bleeding stopped. They gave me birth control pills to take on top of the Mirena, just so I didn't bleed through my wedding dress!!!
I pestered them until they finally listened to me and took it out after 10 months of pure hell! Like a pp said, it was just constant bleeding (not spotting, full out bleeding) the whole time.
TMI Warning!!
Do you know what happens to the downstairs area when it's exposed to constant moisture for nearly a year? It gets all pruny and my skin started peeling off from being rubbed raw and wet for so long. And my new husband and I barely DTD at all. It was easily the most miserable 10 months ever!! And that's saying nothing of the 50+ lbs I put on during that time.