My experience has been absolutely wonderful. I had a Mirena put in July of 2003, had a period that August, and haven’t had one since. Well, once I spotted enough to need a pantyliner for two days in a row, but considering the rate I used to go through the super-absorbency tampons, I’d hardly consider that a period. According to the research I’ve done and my gyn, the heavier periods and cramping are mostly associated with the copper-T type IUDs. They’re not really sure why the copper one causes that, but the thinking seems to be that it’s something about the metal.
I don’t know that I’m necessarily a typical example of a Mirena user, though. It’s like any other bc–some women respond to it wonderfully, some women respond horribly, and most women are somewhere in the middle. Basically, any sort of side effect you can have from Depo, good or bad, you can get from the Mirena. Stopping periods, breakthrough bleeding, water retention, etc., it’s all a possibility. The incidence of side effects seems to decrease over time, though I’m not sure if that’s because the side effects subside or people get sick of them and have the device removed.
As for horror stories, I’ve heard some doozies, but they were mostly from people who were so stupid that they thought just because they had a bad experience that everyone would have a bad experience with it. One woman was begging me to not get a Mirena because she had horrible cramps and bleeding for three whole days and it was so awful she had to have it taken out. (Horrible cramps and bleeding for three days sounds like a pretty normal period to me, but I guess mileage varies.)
Now, I’m not going to lie to you–the insertion and the first day or two afterward ain’t fun. My cervix had never been dilated for anything before, and my uterus is really impressively retroverted, so my insertion was pretty damned awful. There’s a thread about it if you want to read it. The cramps afterward were really, really bad. It took three naproxen, four ibuprofen, and a couple tylenol to get them under control. Of course, I made it a lot worse on myself than it had to be, scheduling it on a day when I had to work. If I’d had any sense, I would have done it on my day off, accepted their offer of valium, come home and taken a big slug of NyQuil and slept through the whole thing. Then I really think it wouldn’t have been that big a deal.