I, too, had Mirena inserted 6-weeks postpartum and have steadily gained weight ever since. I called my doctor's office to ask if that was one of the side effects, and the nurse almost laughed at me and told me that, unfortunately, I couldnt' blame my weight gain on Mirena.
I've gained about 15 pounds during this past year and probably a few more during the 6 months before that. (I've had Mirena for a year and a half.)
Just seeking others' experiences because I am thinking of taking it out and just going back on the pill . . . .