Has anyone implemented this optional rule and have any feedback? Considering a more realistic game, though the risk of losing an appendage has my players a bit concerned. They envision, eventually, a party on crutches with eyepatches, slings on their one good arm, and horribly scarred bodies. Maybe an exagerration, but still...
Hi!
It depends a lot on your style and group needings. 
I play in a (mostly) mature environment in which players take weird rules as a roleplaying opportunities.
You can do this: just go for what you see fit without explicitly declaring the rule as "applied". Don't remove an eye to everyone who reaches 0 HPs or just don't roll everytime for every injury's met condition... Just surprise your players with awesome descriptions on how they hands are cut off.
A concrete example: we were playin' in 4e, was 3 years ago, but my players still remember that episode... A player was messin' around with a hatch in the ground. After lots of "be careful" of the other players and a bunch of clues I (gently) put about that damn hatch, he went on messin' around.
Ok. You deserve it mate...
I Rolled, awesome 20 (gods guided my dice): a hidden blade cut-off the PG's right hand (main hand, obviously), orribly craking is bone and flesh... He was...astonished. "You can't do this..." told his eyes, but I knew that it couldn't argue about that.
The point is: just be concrete and coherent and introduce injuries not as a "rule", but more as a direct consequence of important (or, sometimes just unlucky) choices and scenes.
A player facing its nemesis at the wrong time may experience an insane eye removal with the promise: "I'll bring the other out if I'll see you again around here."
Just go for these type of things, IMHO.
In Rise of Tiamat, one member of the Council is an ex-adventurer who had to depart the field because of a wound that would not heal right. He uses the leg bone of the beast which gave him the injury as a cane.
My HotDQ Paladin had to retire when I became the DM; I took advantage of a near-death (2 of 3 failed Death Saves) curbstomping and said that even full-power Lay on Hands: myself won't fix his injuries. (Mechanically, I shuffled his stats, so this is true.) And I further declared that he was visited by the Red Knight, who told him he was most needed to organize a great expedition. (His background and history say he worked in a Quartermaster Corps in his youth.)
I would only use this rule in combat if I rolled a string of 20s or a PC rolled a string of 1s. Because the PCs are right to suspect that a group of cripples will be un-fun adventurers. But they do make memorable NPCs with a story.
We're trying out a "house rule" that I've seen proposed here about implementing 1 Level of Exhaustion each time a player hits 0, and I've told them that for flavor only, after the battle, if they want to roll on the lingering injuries table and use the result for inspiration, I'm game. Right now, absent playtest, leery about possibly decapitating my gamers, but if they want to say they lost a finger or scratched an eye permanently or got a nice scar somewhere, it'll make the character more gritty. Just won't impose any in-game mechanics.
My plan is to make it a player option. Here are my 0 HP House Rules
- When reduced to 0-HP get a level of Exhaustion.
- If you are reduced to 0 HP and not killed outright, you may instead be reduced to 1 HP (still exhausted) and accept a roll on the lingering injuries chart (DMG272). You may use this option once per long rest.
This gives the player a little more control. Stay in the fight with some cost, or let yourself go unconscious and hope for the best.