Just be alert on any symptoms mentioned here, please! And if you are still breastfeeding, then take into account that in some pamphlets (depending on what country you live in) it is mentioned that Mirena should not be the contraception of first choice for women who breastfeed... I only found out after I had quit breastfeeding my little boy.
I breastfed the entire first year while I had Mirena--no problems at all. I am actually due to have this one taken out and a new one put in cause I had this one inserted in 2/04. I went thru a few side effects when I first got it but given time and patience it all evened out. I spot maybe one or two days a month (when my period would have been) and thats about it...
I hate that there are so many it hasn't worked for but I"m a huge supporter and think it is the best out there!