I had one when DS2 was 6 weeks old specifically because I was told that they don't have an impact on bf. They have progesterone in them like the mini pill which is also supposed to be compatible with bf. The amount of hormones in the Mirena are much less than in the mini pill too.
I had not troubles with bf at all and carried on until DS was 16mths old.
I had my Mirena fitted when DS was 12 weeks old and fed till he was 23 months old.
As Niecie said, its the same hormones as the minipill, but at much lower levels in your bloodstream. Docs at the FPC said that it was the best contraception with breastfeeding if you wanted something long term.