I've had the Mirena IUD for 2 years now and love it. Like the others have said, no cramping, super light flow (if any), no mood swings or weight gain.
Like any BC, there are some it doesn't work for, but the majority of Mirena users love it. You will probably have some spotting for a few weeks (up to 12 weeks) after insertion. It gets old, but does go away. Mine lasted about 6 weeks but it was just minor spotting, more annoying than anything and then I didn't have another period for 3 months! So don't get another peroid at all. Mine's just really light which I love!
Good luck!