I just made my first and only Hellfire Ring; it's got 194 STR, a socket and about 5k health globes and some other worthless stats. Other than strength, it's far inferior to my rare rings. I started another barbarian and gave him the ring with a minor vitality gem in the socket. At level 6, he's invincible --- lol!
I soloed all this on MP7, and it took about 10 runs for 6 keys in each ACT ... which was pretty good. It then took all 6 attempts to get the three different ingredients ... and amazingly I got each of three body parts without a duplicate. Even with all that relative good luck, I was incredibly bored --- like bleeding-from-the-ears bored! I would imagine the odds are that you'd have to make about 20 rings to get one that might have crit damage, crit chance, a socket, vitality, etc.