My doctor offered it to me two years ago and I plan on getting one done for this pregnancy also. It was very accurate and I also made sure to verify the gender was correct during my anotomy scan.
My doctor did it at my request. If you don't have a soft "marker" for downs at the sonogram then most likely your insurance will not cover it. However, your doctor has to request the test, you can't get an order from the lab or just request a kit/blood test for it.
It is very accurate on fetal DNA and I think ranks around 80% for downs, trisomy and other things you can test for (genetics - there is accurate whole book of things they can test for).