Haven't read the entire thread, but the title struck a cord. I really wanted to know about DS in particular and I was 36 and paranoid. After a lot of soul-searching (me and dp) went ahead with amnio. We paid extra (something they only told us was possible on the day) to get the result quickly (2-3 days). Didn't have any money with us except plastic, but they let us post a cheque. Actually got the result the next day before I'd started worrying about it! Then full results and baby's sex a few weeks later.
But, I found out too late there is a private test in london that looks at the baby's nasal bone and has a high success rate. Midwife hadn't told me about the panoply of private tests available which I was a bit pissed about. I was totally ignorant, and felt I was catching up for the first (most important) part of the pregnancy.
I found a website that I found really useful, as I was very aware that once you start testing you are on a conveyor and it's hard to get off it. www.Dipex.org - it has a section on antenatal screening, with interviews with couples who've been through finding out bad news.
Anyway, it's a totally individual choice, I'm still not sure what I would have done if she had turned out to have any genetic conditions, but I did want to know.
Would I do the same again - I would be even older so probably yes, but with even more reservations.