has anyone grown Quinoa?

No personal experience, and in fact it seems like it is still most
common within its traditional range of South America (at high
elevations). See for example:

http://www.farmradio.org/english/rad...6script_en.asp

if you pick the leaves for cooking, will the plant continue to grow
without going to seed?

Well, I can give a general answer - if you want to pick leaves and let
a plant keep growing (lettuce being the classic example), you just
pick some of them (I start with the bottom/outside), but not so many
that the plant is seriously set back. For some plants a scissors or a
knife will keep you from damaging the part you aren't picking (others
you can use more of a fingernail or pinching motion). If you want to
prevent it from going to seed, often harvesting the middle (apical)
leaves will slow that down.