I have one and it was not fun having it put in - I screamed out and it was a bad 5 minutes of my life but at least it wasn't child birth so for that I'm grateful. I took some painkillers an hour before they put it in as I didn't want to wait for them to take effect. I felt bloated that evening and by the next morning I couldn't even tell I had one (not even any strings).
I've not had any periods since, although I'd not had any periods for many years before thanks to Depo and the pill... and no I don't recommend Depo as I think that's caused my borderline osteopenia.
As for wait gain or slowing loss, I couldn't say. The amount of hormone in it is supposed to be tiny - way lower than Depo as it's localized rather than going round your whole body and I lost while on Depo. I was out of the honeymoon period by the time I got my Mirena.
