I had Mirena for 4 years after my 3rd child. I loved it. No cramping. No periods. Nothing to remember to take. Although hubby said he could feel the cord, he said it didn't really bother him.
Then, I got pregnant. When I looked back on it, I did get one period completely out of the blue 3 months before I got a positive pregnancy test. I think I had a very early miscarriage then, too. The IUD had imbedded itself into the uterine tissue. The pregnancy wasn't viable, so I had a D&C to have it removed. According to the doctor, it was REALLY imbedded into the tissue. When I was going to appointments for bloodwork, etc. after the positive test, I had many nurses and staff tell me everyone in the office had the Mirena, so they were all nervous. Several months later, at my general practitioner's office, when she heard me tell my story, she was so shaken, she had to keep apologizing for being distracted. So, it is rare to get pregnant - but definitely possible.
As you've seen in the posts, some people love it. Some hate it. Pretty much just like any form of birth control. If your insurance covers it, you could try it. If it doesn't work for you, then you can go with your back-up plan. I have heard many women use the Nuva Ring or other IUD's with less hormones in them.
Good luck!