A.P.
I have had the Mirena for a little over a year and I love it. I get very light periods and don't have to remember to take a pill or something. When I had mine inserted they told me to take some medicine before I arrived and it basically felt like cramping. Overall I have been very pleased with mine. You should research it a bit, Wikipedia has a good entry on it. There you will be able to find tons of information including how it works to prevent pregnancy.