Unfortunately I don't have the 20mm,...wish I did! However, I am currently using about 8 lenses in FD fit, both actual Canon items as well as a couple by independant makers. I aquired the adaptor at the same time as the NEX 3 because FD glass was one of the reasons for going to the NEX. The adaptor was the cheapest one I could find on Ebay being about £12 but it has been quite adequate in use. It has a 'stop down' ring which closes the aperture to that set on the lens. You need to be mindful when fitting the lens to the adaptor that this ring is set 'open' otherwise you can't operate the ring/apertures. Other than this there are no problems and stopping down is not always neccessary as it's possible to focus at chosen aperture up to about f8 using the 'manual focus assist' feature.
This is the adaptor I use fitted with Canon 70 - 210 f4;
I use Canon FD 35-70mm most of the time, I also have a Vivitar lens which is FD fit. I love it, I've had no bother with the adapter as long as you put it on open then lock it in place, as Bugleone says. Adapters are dead cheap on ebay and well worth it for the range of options it will open up.