I had a non-hormonal IUD put in last summer. The first few periods were VERY heavy (super plus tampon and a pad that I had to change frequently). By the third period, things were much better. I would have a very light period for a few days, one or two heavy days and then a few light days. My period seems to last a bit longer than it did before. Before the IUD I always had a very predictable and normal period. I am still predictable from a number of days stand point, but the flow is just odd.
I have not experienced any increased cramping or PMS symptoms. I think I tolerate this better than the pill because that made me very moody and it is so nice not to have to think about anything!
If you do get one, be sure to bring a pad with you to the office and take some ibuprofen before you get there. It pinched a bit for me and I had some spotting for a few days after.
Good luck with the new baby!