I am 32 and nulliparous, so I was pretty nervous about the pain. I took 800mg of ibuprofen an hour before the appointment. The worst part was having the local anesthetic done. After that it was pretty smooth sailing. She did have to dilate my cervix a bit, because apparently mine is curved! There was some pain with that, but the nurse was super helpful in chatting me up in an attempt to keep me distracted.
All in all, it was a piece of cake. It was just a little more painful than having my regular pap done. I am in no pain at the moment, but plan on spending the evening with a heating pad and pain killers just to be on the safe side.
I'm so happy I did it! I've been wanting a Mirena for years, and finally made it happen. I'll try to remember to post a follow-up later!