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.
Wow! Those are interesting stats. I'm not really surprised about the barrier methods - I imagine there's a fair amount of 'user error' in falling preggers with those but am very surprised at the surgical stats vs the Mirena.
I've never been a fan of the concept of an IUD... it's personal I suppose just like there are loads of people who hate condoms and DH and I have happily & successfully used them for years... even found them vaguely 'fun' at various points. But DH had a vasectomy last year and wow, it has been fabulous. But as the OP said, yes, you need to be comfortable with the permanency of it.