I had a positive AFP, with amnio recommended - but I had planned on adopting special needs, so I didn't have the amnio. My son is perfect.
I'd also like to direct you here: http://www.ceri.com/dixie.htm
My older sister was Madison's teacher for some time, she said her biggest problem was the fact that she is deaf - not ds.
That being said, I'd also take into account your age and your husband's (with advanced age comes higher risk), and the willingness to risk a miscarriage of a healthy baby to find out if the baby you're carrying has ds - AND whether or not you'd terminate if the test is problem.
My logic - if you would terminate, and you're over 35 or your husband is over 50 - and you know your fetal age is correct, then it's probably a good idea to go ahead and test.
If the idea of a miscarriage makes you want to sue for malpractice, then it's probably a bad idea to test.
If your dates may be off, the AFP is probably a false positive.
If you would not terminate, but want the information ahead of time to "be prepared", realize that later on in the pregnancy they can do an ultrasound and measure the nuchal fold and a 3d visual of the face can give a very good idea of what, if anything, you may be dealing with.