ECHOnet has been distributing this plant for a long time. Here is what they have to say about it:
“I would consider chaya to be one of the five most important underutilized
food plants ECHO distributes. I give it this rank because of its ability to
thrive in both arid and rainy regions, its little need for care or extra
fertility, its lack of insect or disease pests, its high production per
square foot, and the exceptional nutritional value of its cooked leaves.”
They have a 16 page PDF about it here:
http://echonet.org/repositories#18:d:Chaya
The PDF is just one part of their "Ideas for growing food under difficult conditions - From Amaranth to Zai Holes ",
which is this:
http://www.echonet.org/content/AtoZ