I didn't have it with daughter, but only bc I knew I was already high risk (1:11 for trisomy 13, 18 and 1:17 for Down Syndrome) due to age, so I did the scan part of the screening and then the MaterniT21, which is a blood test that tests the fetal DNA for chromosome differences. It gives you a positive or negative, and not a ratio like the screening.
Personally, if I hadn't already been high risk I would have had the screening. I'm one of those people who like to have all the information I can so I can make better decisions. And I'm not talking about decisions regarding termination, bc that's not even an option for me. One of our local newscasters posted a blog about her daughter's birth and has continued blogging. Anyways, this newscaster and her husband were both in their 20's and healthy. They had all the usual ultrasounds during pregnancy with no red flags. She gave birth and her daughter had Down Syndrome. She said she grieved the first few days after birth; not bc she didn't love her daughter to pieces, but she grieved the life she had dreamed for her daughter. She said she spent those first days doing tons of research and gathering information. She said all the family and friends who had gathered at the hospital for the birth had no idea what to say. She went on and on, but in the end she said she wished she had known beforehand so she could have enjoyed the birth and days after instead of being in shock, not knowing what to do and so forth. I know I'd be the same way. Plus, what another poster mentioned, I'd want my doctors prepared so that baby would have the specialists needed if there was something wrong. This would have meant I'd have had to deliver her at different hosptial than the one I had chosen. But that's jsut me.
It's such a personal decision. My sister is totally opposite of me and is more of a 'go with the flow' type person who deals with something when/if it happens and doesn't worry. The screening wasn't routine way back when she was pregnant, but I'd bet that if it was she would have declined.
I would like to point out 2 things: one is that this is just a screening. It can't tell you if anything is actually wrong with baby. It doesn't give false positives bc it doesn't give a positive or negative result. It's just a ratio. Any sonographer that tells you that your baby will have a disability due to this screening shouldn't be doing ultrasounds!
the second thing to keep in mind is that there are now noninvasive tests that can give you more definitive information (like the MaterniT21 I mentioned) You don't have to do an amnio anymore.
Good luck with whatever you decide.
I would not have terminated if there were any issues detected but it would have changed my birth plan. I want to have the baby at a free standing birthing center, if there were issues I would want to prepare myself (and have my midwives be prepared as well) and give birth at the hospital instead. So anyway, that's why I chose to have it done 
