Pros and cons to Mirena...
I was fitted with the Mirena in February 2008 before ever having a child. It was inserted while not on my period and was only a tiny pinch of discomfort. I did have pretty heavy cramps the rest of the afternoon that day but afterward, no discomfort to speak of. I don't recall any spotting or periods after insertion and didn't gain any weight. I'm terrible about remembering to take a pill daily so Mirena was the BEST birth control IMO.
As for sex, I did find it uncomfortable in certain positions when my DH would penetrate more deeply and bump the IUD but this would be on a case-by-case basis depending on the couple since cervical position and penis length would be factors to consider...
I started getting my period again about two years after insertion. At first I thought it had been bumped out of place or fallen out but the doctor confirmed it was still where it needed to be and was just "one of those things". It didn't really bother me. They were cramp-free periods that came every 29-33 days, same as before Mirena.
I had the Mirena removed in February 2011 in order to start TTC. It was removed while not on my period and was completely painless. Less than 30 seconds and she was done and I hadn't felt a thing. Since I was still getting my monthly period before removal, it continued on the same pattern as before. I started charting my BBT immediately after removal and found that my temps followed the "normal" pattern.
So with all of this information, I would say that Mirena is WONDERFUL.
However, I did test with low progesterone levels when TTC and based on communications with several other ex-Mirena IUD users who also have problems with low progesterone, we attribute this to the Mirena IUD. It's like our bodies have forgotten how to produce its own progesterone which is needed to sustain a pregnancy so while several of the ladies I chat with have suffered miscarriages from the low progesterone, I was lucky enough to be tested early and start a progesterone supplement right away which resulted in a healthy pregnancy.
As much as I loved Mirena, my DH and I have decided not to use birth control between children so as to avoid future progesterone issues...