Dear K.,
I can't give you any advice on endometriosis but IUD has been my birth control method for nearly 10 years now.
I had the T-copper for 6 years, I think it would be easy enough to check if you are allergic to it or not. It's best for women who have already given birth, for women who haven't, can cause painful menstruations.
Then I had the Mirena for 4. It has a smaller concentration of hormones than the pill and is estrogen free so you don't have to deal with the estrogen related side effects.
I didn't have any problems with it. Got my period regularly, free of pain, didn't experience bloating, weight gain or any other problems. Of course, each woman has a different experience and you can't find out until you've tried. The only downside is that inserting and removing and ID is not party so it's not like you can try several models easily. You need to consult with a specialist and providing that IUD is at all an option in your case, then figure out what is the model that'd work best for you and try it.
If you haven't had one before this is how it goes: Prior to insertion the doctor will make a test to rule out any infections, then you need to have your period for the insertion (it's easier since you are dilated). If the doctor has experience in inserting IUDs, and I recommend you look for one that has, the insertion takes only a few seconds, you might feel some discomfort or a pinch but totally tolerable. With the Mirena you might experience some irregular periods at the beginning or even skip some but eventually it starts coming regularly, in my case, lighter than without the mirena and free of pain. You need to check if the strings are in place from time to time, your doctor will show you how to do this. I have a confession: I did not check mine once in 10 years and so far so good.
For the ladies out there: IUDs are best for women in stable relationships and with only one sexual partner. They are not advisable for other alternative types of relationships.
The return of fertility is almost immediate when you remove the mirena and for that I can vouch myself ;)
You can check more info about the mirena in the link below.
http://www.mirena-us.com/whatismirena.html?C=&c=
This model worked very well for me but as I said, each woman is different, so don't go and choose it based only on my recommendation, do your homework.
M..
Married for 11 years and with one 7 month-old adorable, good humoured and kind baby boy.