I also had two amnios with my first pregnancy, and a CVS with my second. The first amnio was at 16 weeks, and requested due to "advanced maternal age" since I was 37yo at the time. It just so happened that a friend had had one about a year before, so she told me what to expect, and I was glad she did!
The needle is HUGE. Long, I mean, and probably kinda thick. But very long. I'm not fond of needles, and my doc (whom I adore) had it in her hand, sort of bouncing it around as she explained the procedure to me. Then she realized what she was doing and said, "Oh, gee. You probably didn't really want to see that, did you?"
You will feel three sort of electrical pulses (that's the best I can describe them) as the needle punctures through layers of tissue, the final being the uterine wall. That is perfectly normal. Once the needle is in, it will take about one minute to gather the fluid, so I slowly counted "one-one thousand, two-one thousand..." and she was done right on 60.
I had a tender spot where the needle went in afterward. She advised me to go home and rest the remainder of the day, though since I was in an office job, I probably COULD have gone back to work. But I went home as advised.
I loved knowing that the baby was healthy. Additionally we found out the gender, a nice little perk so we could plan the nursery better and stock up on boy things.
PS. My 2nd amnio was for the same reason as another poster's, to test lung maturity right before a c-section delivery. My uterus had "issues", so they wanted to make sure to deliver early enough that I did not go into labor, so the test was at 37 weeks, and we delivered two days later.