Don't worry, this entry is spoiler-free. 
 
If you own a DS and you have not played 999, it is now your civic duty to go buy it.  It's one of the best-written games I've ever played, with an excellent cast, a suspenseful atmosphere, and gameplay that really respects the intelligence of the person playing it.  The game's little-known, but it's easily worth the $35 price tag.  If you've played Ace Attorney, Professor Layton, or Hotel Dusk, you'll have a good idea what you're getting into.  It's point-and-click adventure, with lots of dialogue and puzzles to solve.  While I was initially skeptical, it earns its 'M' rating, a rarity for the DS.  Off the top of my head, the only other M-rated DS game is Grand Theft Auto, which has nothing in common with 999.  The story is a little tough and hard to follow, and some of the math in the game really requires that the player know how to think, but I still feel this is a game that everyone who considers themselves gamers should pick up. 
 
That said, it's difficult to discuss the game without blowing all the major plot points.  Everything in the story just fits together so damn well.  However, there is one aspect of the game that I haven't seen done in a game before, and that's the way 999 handles multiple game endings.  They aren't that uncommon in games...Heavy Rain comes to mind first, and I know the Shin Megami Tensei series loves to do it.  But in Heavy Rain, I still felt that one ending was enough for me.  I beat the game, and it was my own personal experience.  Sure, it's interesting to see what happens to other people, and it's fun to read up on how all the different choices you make effect the game...but in the end it's still the same experience for me.  In SMT, multiple endings are more of a gimmick.  You play the game this way, it ends a certain way.  That's all there is to it. 
 
However, the multiple paths in 999 serve a very different purpose.  Instead of each ending being a definite stopping point for the player, they serve instead as instruction.  The different paths you take reveal different facts about the characters, and depending on what conversations you've had previously, your next encounter may go a lot differently.  There are really only two substantial endings, and one only serves as a bridge to the final, true ending (which I somehow managed to get through on my first playthrough, but I lacked the necessary information to actually finish the game).  The other endings are basically glorified "game overs", though one in particular is genuinely creepy and unnerving, and worth playing through just to see it for yourself. 
 
It really says a lot that breaking up the narrative structure like this can have such an impact on the game's experience.  I felt bored with games like Red Dead Redemption or Assassin's Creed: Brotherhood thanks to that simple "seen it all before" feeling, even if the game is in the wild west.  But when was the last time that getting multiple endings told you more about the world?  Shadow the Hedgehog?  (not recommended you play that one)  My biggest complaint with games like Ace Attorney or Hotel Dusk is that they're ridiculously linear, but 999 tells a compelling story despite, and because of its branching paths.  Otherwise it'd just be a well-written, but unremarkable adventure game.  Pursuing multiple scenarios as if they were all equally important as the last just strengthened my bond with the characters, including seeing what would happen if I decided to just screw everybody over. 
 
Oddly enough, my only complaint about 999 is that I wanted to see even more of it.  It doesn't end on a cliffhanger, but the potential for a sequel is there, and the director is fully ready to make another one, possibly for the 3DS.  I read a long interview with him, and it pleases me to know that there are people like him who take video game writing seriously.  He openly discusses his thought process and discussions, which make me all the more excited to become a writer myself.  The thing is, he'll only make a sequel depending on how well-received this game is.  So go purchase Nine Hours, Nine Persons, Nine Doors right now!

Don't worry, this entry is spoiler-free. 
 
If you own a DS and you have not played 999, it is now your civic duty to go buy it.  It's one of the best-written games I've ever played, with an excellent cast, a suspenseful atmosphere, and gameplay that really respects the intelligence of the person playing it.  The game's little-known, but it's easily worth the $35 price tag.  If you've played Ace Attorney, Professor Layton, or Hotel Dusk, you'll have a good idea what you're getting into.  It's point-and-click adventure, with lots of dialogue and puzzles to solve.  While I was initially skeptical, it earns its 'M' rating, a rarity for the DS.  Off the top of my head, the only other M-rated DS game is Grand Theft Auto, which has nothing in common with 999.  The story is a little tough and hard to follow, and some of the math in the game really requires that the player know how to think, but I still feel this is a game that everyone who considers themselves gamers should pick up. 
 
That said, it's difficult to discuss the game without blowing all the major plot points.  Everything in the story just fits together so damn well.  However, there is one aspect of the game that I haven't seen done in a game before, and that's the way 999 handles multiple game endings.  They aren't that uncommon in games...Heavy Rain comes to mind first, and I know the Shin Megami Tensei series loves to do it.  But in Heavy Rain, I still felt that one ending was enough for me.  I beat the game, and it was my own personal experience.  Sure, it's interesting to see what happens to other people, and it's fun to read up on how all the different choices you make effect the game...but in the end it's still the same experience for me.  In SMT, multiple endings are more of a gimmick.  You play the game this way, it ends a certain way.  That's all there is to it. 
 
However, the multiple paths in 999 serve a very different purpose.  Instead of each ending being a definite stopping point for the player, they serve instead as instruction.  The different paths you take reveal different facts about the characters, and depending on what conversations you've had previously, your next encounter may go a lot differently.  There are really only two substantial endings, and one only serves as a bridge to the final, true ending (which I somehow managed to get through on my first playthrough, but I lacked the necessary information to actually finish the game).  The other endings are basically glorified "game overs", though one in particular is genuinely creepy and unnerving, and worth playing through just to see it for yourself. 
 
It really says a lot that breaking up the narrative structure like this can have such an impact on the game's experience.  I felt bored with games like Red Dead Redemption or Assassin's Creed: Brotherhood thanks to that simple "seen it all before" feeling, even if the game is in the wild west.  But when was the last time that getting multiple endings told you more about the world?  Shadow the Hedgehog?  (not recommended you play that one)  My biggest complaint with games like Ace Attorney or Hotel Dusk is that they're ridiculously linear, but 999 tells a compelling story despite, and because of its branching paths.  Otherwise it'd just be a well-written, but unremarkable adventure game.  Pursuing multiple scenarios as if they were all equally important as the last just strengthened my bond with the characters, including seeing what would happen if I decided to just screw everybody over. 
 
Oddly enough, my only complaint about 999 is that I wanted to see even more of it.  It doesn't end on a cliffhanger, but the potential for a sequel is there, and the director is fully ready to make another one, possibly for the 3DS.  I read a long interview with him, and it pleases me to know that there are people like him who take video game writing seriously.  He openly discusses his thought process and discussions, which make me all the more excited to become a writer myself.  The thing is, he'll only make a sequel depending on how well-received this game is.  So go purchase Nine Hours, Nine Persons, Nine Doors right now!

I absolutely agree with you.
 
I played through 4 times in a row, and all 4 endings I got were amazing.

I played this game so much I didn't sleep for like three days god damn it. 
 
It really does handle the endings really well. Although i kind of fell into finding the truth, there is no reason that someone shouldn't be able to finish this game without a FAQ. It makes it pretty clear what factors lead to which results and how you can manipulate those outcomes. 
 
I don't think the way 999 handles its endings would necessarily work for most games, but it is certainly worth investigating as it is a hell of a lot more rewarding than just going from point A to point B.

I agree, this game is amazing. I've finished the game twice now and I'm already looking forward to my next playthrough.

Yup, it was absolutely terrific. This as well as Ghost Trick are prime examples of what more developers should try to do. I'd love to see a similar type of game appear with better and higher-res art on consoles.

I don't think there is a real need for a sequel, but hell I'd kill for more games like 999. I believe that there isn't a game that is localized better than this one, the writing feels like it was originally done in English.

I'm incredibly excited to see what the other endings have in store. I've just finished my first playthrough (the ending I got was what you see in the second icon in the Save/Load screen) and immediately restarted. I don't do that for many games anymore these days since I'm old, decrepit and busy.