Yes, Hellfire Rings are absolutely worth it. I even replaced a trifecta ring. The main stat is higher than pre-2.0.1, and you have a high chance of "good stats" since all the crap (poison resist etc) moved to secondary.
Good question and response. I wasnt very heavy into Vanilla D3. I do have 8 machines, and after all the keywardens Ive killed in 2.0 Ive yet to see a keydrop. Currently playing a lot more to make a viable ubers toon.