I've had the mirena iud since 2011 and haven't noticed any bad side effects except on the day my period comes I have really bad cramps. It only lasts for a few hours that one day. Other than that I haven't noticed anything else out of the ordinary.
The insertion is really painful but once it's in you're done. And majority of women don't get a period while having mirena because they are totally lucky. But my uterus apparently doesn't want me to have it as easy so I still get a period regularly but it's substantially lighter than it was before. That's why I got it was to make my periods lighter and it worked really well. It expelled once and I had to get it replaced but it was not as big of a deal as people make it out to be. I'm never going back to heavy periods ever again so I'm sticking with mirena until I hit menopause.