Turns into materials comparable to a rare. Does not yield any cool legendary stuff like organs, if you think this might happen. It also doesn't give a brimstone (not sure if it CAN'T give a brimstone or there's just a small percentage, but there was actually a bluepost saying they don't want HRs turn into brimstone farming resources).
Hellire Rings can also roll trifecta. They can even roll quadfecta PLUS they always have 170 main stat, so essentially you can have a quinfecta HR with 35% bonus XP, pretty awesome, right? But yes, the HR is mainly for people who're not p100 yet. To be honest, that's like 99.9% of all players (and even those who have a p100 char might use their HR for a twink).