I had the mirena inserted a few yrs ago when it was still considered a "specialty" form of contraception as I had lots of period issues.  The insertion was excruciating.  I could not sit up for 2 hrs.  I had cramping during non-period times the entire time it was inserted.  I gained weight and had edema (swelling) in my belly, breasts, feet and legs due to the progesterone that is released directly into my uterus the entire 2 yrs I had the contraption.  When I tried to have it removed, the string had wrapped itself around my cervix, which made the removal painful as well.  I do NOT recommend the mirena.  There is no need for the progsterone to be released into your uterus and the cramping does not get any better with time.