I had the Mirena inserted in May after my daughter was born in March, and I have been VERY pleased with it. It was uncomfortable to get it put in, but not horrible, and I had some cramping and spotting that afternoon, but nothing since. I can't feel it and have had no side effects. I have yet to have a period, though I just stopped breastfeeding 2 weeks ago, so that could play a part.
I chose the Mirena over the Paragard because I had gotten several recommendations from friends for the Mirena. No one I knew had had the Paragard.
I love not having to remember to take birth control. Also, my husband and I are *pretty* sure that we are done having kids, but I didn't want to do anything permanent yet, so this is a nice inbetween step, since it is good for up to 5 years.