It depends on a lot of variables, like the shape of the woman, the way the doctor left the thread positioned as it emerges from your cervix, and the unique way the couple functions together in intercourse.My b/f is pretty sensitive when it comes to that sort of thing - feeling weirdness inside, I mean. I had used the NuvaRing before and it caused him a great deal of discomfort, which is partly why I have opted for the Mirena. At any rate, he can DEFINITELY feel the string from the Mirena when we have sex, and he seems to really enjoy it!! The orientation and length of the string stimulates his most sensitive area, depending on the angle. :)
I just had mine inserted two weeks ago. I have not had enough time for my body to adjust, so I can't make any fair judgments on the outcome just yet. I will say, however, that I am spotting right now. And not just regular cycle spotting.... It is a bizarre rust-colored sludge, about the same consistency as hair styling glue. I have had temperature fluctuations, mostly hot flashes and very low grade fever. I have been quite irritable, too, and impatient. My doctor instructed me to continue my previous birth control (Yaz) for one additional cycle until after my Mirena follow-up appointment to make sure that I am protected against pregnancy. Some of my current side-effects may very well be caused by the combination of hormones between the birth control pills and the Mirena, since they are putting a higher level of hormones into my body than what it was accustomed to. If these symptoms have not subsided in three months, I might revisit these issues with my doctor and see if he can offer any suggestions.
As far as the insertion procedure.... Mine was especially complicated, as I have never been pregnant, and apparently my internal features are especially "petite" (borrowing my doctor's word), and as such the insertion required additional care and effort. The amount of pain and discomfort I experienced is likely to be misrepresentative of a typical insertion procedure.