B.W.
I have a Mirena IUD, and I LOVE it. I've had it for a year now, and I have not one complaint about it. The first month I had it in, I did spot off and on, and around the time of my period I had wicked cramps. But the 2nd month I spotted a bit around my period, and by the 4th month I didn'th ave a period at all. A year into it, I haven't had a period in 7/8 months, no cramping or spotting, can't even tell its there.
I had very light periods before my boys were born. Even between them, my periods went back to normal, but after my youngest was born and weaned at 18 months, my period was awful! Cramps, and very heavy, I was tired and had a headache.... awful.
I got the Mirena for birth control, but honestly if we never choose to have more children and my husband has a vasectomy, I will probably continue to use it for its period control. I didn't gain weight either. In fact I've been loosing weight with diet and exercise very steadily.
Insertion was the most uncomfortable, it was like a big contraction, and then its over. It needs to be placed while you are on your period, which is kind of skeevy but really wasn't that bad, and it was very quick. My doctor had it in in less than 30 seconds.
Good luck in your choice, but I have had a very positive experience with mine.