Blah,blah,blah, it's not real, it's not pure, it's not name brand, where can I buy the real stuff, etc....
Case in point:
Effect of Impurities on Propane Refrigeration System | Campbell Tip of the Month
The main idea is this:
If you're a pro, your supplier can get it. You should know where to look. Is it cheap? Not from Airgas, National, Linde, etc. This stuff is not unknown, it is trying to be denied a future in America. Industry has been using propane, propylene, butane, ehtane, and ammonia as refrigerants for 100 years plus. Their propertes and usefulness are proven. Only since 2010 have they been considered seriously as replacements for traditional, halogen-based refrigerants.
Should you use it? The answer is it depends. Probably not in a customer's heat pump. Not yet, anyway.
Linde has their own line of natural refrigerants. Take a look here:
Gas2010 | Natural Refrigerants
"Linde" brand R290 is Care 40, and guess what? Just about the same price as R22.
IMHO, Worthington or Bernz-o-matic Fat Boy cans are at least 95% r290 (HD-5 grade), pre-blended with up to 5% of propylene (r1270 aka Care 45), butane (r600), and/or ethane (r170) as an added bonus. If there's much more than .001% of non-condensible gas or water in the mix, it messes with the process at the propane plant and it won't liquify in their still. The only thing you have to worry about is the "stink oil", which a standard catch-all will catch on the way in. For $3 a quart, that's what I use with great success.
If I were a scientist doing precise lab experiments for the government, I would source pure R290 and eliminate all doubt. However, my labor would far outweigh the $20 per pint expense of the propane. But Uncle Sam would get his money's worth.