Remember that there is a natural bias that people have a tendency to write about things they are NOT happy with, and not write when they are happy with something, unless specifically asked like you have here. I asked my Ob about mirena and her patients' experiences, and she said most patients love it, she puts in a lot of them and rarely takes one out early because of side effects. It does happen, but if you read much online, it might sound like everyone is unhappy with it, and I don't believe that's true.
That in mind, I have mirena, I had it inserted at my 6wk postpartum appointment. Not only did I not gain weight, but I have lost all my baby weight in the 4 months I've had it in, no moodiness, and no change in sex drive (although that naturally takes a hit postpartum and with breastfeeding). I do have random spotting and they say that can happen for a while (6 months or even a little longer). But it's not enought to interferewith life at all (a pantyliner is enough to handle it). I like it because I don't have to remember to take anything, I'm nursing and it's compatible with breastfeeding, and I am looking forward to lighter periods (haven't had one yet b/c of breastfeeding, but I'm hoping).
I would say that if you've been on the pill in the past and it was ok for you, the hormones in mirena will be fine. If you've never handled hormones well, in the pill, etc, mirena probably isn't for you.