I don't have an IUD myself, but have seen many people with them. It's not 100% effective for birth control, but awful dang close (better than pills b/c you don't have to remember to take it). Most women will have lighter periods with the Mirena (heavier with Paraguard).

STD infections can be worse with them (instead of staying around the cervix it can go up through the cervix to uterus and ovaries) - at the clinic we just make sure there is no recent history of STDs, and are in a stable relationship.

The main reason doctors don't like to insert IUDs into women who haven't had children is because of a more difficult insertion if the cervix hasn't been dilated. It can be done, it's just more uncomfortable. The effects of the IUD are readily reversible when removed. The patient just has to check to make sure the strings are still there - rarely, the IUD can fall out on its own.

Hope you figure out what will work best for you.