I just had my amnio today and it wasn't that bad.
I have a complete anterior placenta, but they still found a good-sized fluid pocket to draw from and didn't have to go through the placenta. The needle they use is a .22 gauge, which is the same as for a blood draw in your arm (actually smaller than the one they sometimes use!). The worst part was when the needle went through the uterine wall, it causes some cramping that was fairly uncomfortable. But that lasted maybe a minute, minute and a half, and then it was over. The nurse and doctor talked me through the whole thing and were super nice and calm throughout.
So try not to worry, the worrying about what it will be like was actually worse than the procedure itself

I'm taking it easy on the couch for the rest of the day - I've had some mild cramping but no bleeding whatsoever.
Anyway, that was my experience, just thought I'd share.