Copper IUD

So I've been on birth control since 7th grade because it was believed that I have Endometriosis. Now I hadn't officially been diagnosed with it and had to go off of it for a few years(the best I felt minus more cramping). The most recent birth control they put me on has made me gain about 20lbs due to the added hormones. I am currently considering the copper IUD and would like opinions. Either good or bad. Thanks ladies!