Just to put it into perspective if 2000 women used a mirena for one year less than one would fall pregnant whereas if 100 women used a diaphragm somewhere between 4-15 would fall pregnant, which is similar to condoms. Mirena has a better success rate than getting a vasectomy or getting your tubes tied.

In the seventies they had a big problem with copper iuds, they got a bad name, many babies were born holding them but it is very different now

A copper IUD only prevents implantation but as Jennifer13 points out a mirena prevents pregnancy in different ways including changing the cervical mucous to make it hostile to sperm which prevents fertilisation.

Side effects are really the main reason people discontinue different contraceptives and as they go the Mirena is streets ahead of the depot injection or the pill as the levels of hormone reaching your system are less...BUT some women still get side effects... I couldn't tolerate a mirena for the local side effects and I was allergic to the plastic it was made of. Highly unusual but a bummer. I still recommend then because when they work they are wonderful.