I've had the Mirena IUD in for two years now and love it. As pp said it's a little unnerving at first when your periods start to go away, but once you get used to it, it's great! It's like being a little girl again, not having to worry about when you're going to start your period.
There's a little discomfort with the insertion, but I wouldn't even worry about it because it's just a one time thing and is nothing for 5 years of effectiveness.
In terms of the hormone in Mirena, I'll say that I had terrible migraines on every kind of bc pill including the super low dose ones and on the nuva ring and then pregnant and when my son was a newborn. I have had just one in the last two years and I'm sure that was about stress not hormones. I've also had no problem losing weight while it's been in. I'm not a dieter, but I started going to the gym and easily went down a couple sizes, so from that perspective I think it just impacts different people differently.
I chose Mirena over copper because there is some risk of copper IUDs implanting in the uterus and that just freaked me out, plus there's more risk of side effects like ongoing spotting and cramping.
Hope this helps.
T.