I just got a Mirena 3 weeks ago (I am 11 weeks pp right now - so 8 when I got it). So far, so good. The insertion was super easy. My doctor had me take some motrin ahead of time, but the cramping was very minor afterwards (I actually wouldn't have known about it unless my doc had told me I was cramping while I was still on the table).
Best of luck ... hopefully you will get some responses from others who have had it longer. Oh - I have a friend that has been using this type of IUD for over 10 years and she loves it. She actually stopped having periods altogether (but she is also very thin with very little body fat ... so I'm not sure what caused that).
arancini