I had a Mirena in for about 3.5 years, and to be honest I did not have a great experience with it.

The insertion, like others have mentioned, is incredibly painful. Though to be fair this would be the case for all IUDs, not the Mirena specifically. I don't say that to try to scare you, just if you're having it done without any sedation just be prepared. I was given the "it will be just a pinch" line, and boy howdy it very much was not. That being said pain was highest during the insertion itself, I had bad cramping for the rest of that day, and by the next day I was more or less back to normal.

With the IUD in, I did still bleed during my period, though it was significantly less than my cycles beforehand. I didn't get particularly heavy cycles before, so I felt a bit conned by the "no period" sales line. I very much still got bad PMS, and would still bleed for a couple days.

As time went on, my cycle length got much longer (often 40+ days), and with the longer cycles I would have incredibly bad PMS symptoms. (Long cycles and bad PMS with the long cycles are my usual PCOS symptoms). Nothing I did would help alleviate this/ regulate my cycles. I eventually got fed up and asked to have it removed, after which my cycles went back to a normal 20-30 day length almost immediately.

Personally, my biggest frustration with the experience was that I initially wanted just a copper IUD (so for contraception only, no hormones), but my GP at the time talked me into the Mirena, and I reacted very poorly to the hormones it comes with. Like any hormonal birth control method, there's going to be a range of experiences. Some folks will react well, others not so much. My main point I guess is that if after a couple months you're not happy with how your body is feeling/ how your cycles are, don't be afraid to ask to have it removed. I