I had one for about a year, it was great in that my periods were next to nothing, but I did bleed every time I had sex, which isn't great. I did feel that I was suffering with bloatedness and nausea and also mood swings that I had never had a problem with before. I haven't put on much weight - and I think there's as much chance that's to do with what I was eating! I then looked on the internet and found some women chatting on the forums about the Mirena coil - mostly in a negative way. I decided to have mine removed - but fell pregnant within 10 days (despite having polycystic ovaries and having had to have Clomid to conceive my first 2 children!) - so don't know if it has made any difference at all (still have the backache, nausea and bloatedness anyway - and far more weight gain!).
I am slightly sceptical about the forums, in that you can find yourself with all sorts of symptoms if they're suggested to you (I think I may have fallen victim to that!). Backache, bloatedness, nausea, weight gain etc have so many different potential causes and aren't necessarily all related to the same issue. Everyone's different - if your consultant thinks it might work for you, it sounds worth a try? Good luck.