I got a Mirena 2 months ago - Best thing I have ever done! I took 2 ibruprofen and 2 paracetemol about half an hour before I went in, as advised by the doc - I too had very very heavy periods, to the point where I was wearing massive night time pads and having to change them every 2 hours, they were so painful it was ridiculous!
So I had been thinking about it for 2 years (Since DS was born) - sounds perfect for me as DS was no planned and we don't intend to have any more children until our finances are sorted (so atleast the 5 years we are covered by the mirena)
To the details - It was a bit weird, apart from pregnancy I've never had anything looked at down there by a doc - I was very nervous, but the doc I saw was fab, she was very straight forward and asked if I wanted any other tests doing at the same time but I was so nervous I said no, she then asked me about my medical and family history then we got down to the knicker stripping, she told me to just relax and at this point a nurse came in to help out, she was very funny and actually did the trick to calm my nerves -
It did feel very weird, she clipped my womb and then within roughly 10 seconds it was all over, the nurse passed me a sanitary towel, I got dressed we said see you soon and I drove home - It was sore for a few days and the first period after that felt a bit like a contraction but it as literally one contraction type feeling then a very light bleed for about 6 days, for which I used normal sanitary towels which was very nice ha ha
Now, it does feel weird but if your periods were anything like mine then DO IT, I cannot stress enough how much better this has made my life and its only been 2 months!