I've dabbled in visual novel games a little (Fate/Stay Night being pretty much the only one) and have been an Adventure game fan since the old LucasArts games back in the 90s.
One of my favourite series of all time is Phoenix Wright. But, I haven't played a game where the story and dialogue are as top-notch as they are here.
999 has been a wonderful surprise for the DS. Extremely dark (though not without some good humour that made me laugh outloud at points), with great writing in both dialogue and prose. Swearing was used very effectively to help emphasis the emergency of the situation. This is not a game for kids. Even though the game doesn't show much beyond a few bucketfulls of blood, the graphics descriptions of some of the gorier scenes will make you cringe, even if you have no issue with movie violence.
Also, the game features 16 puzzle "rooms" not unlike flash games like "The Crimson Room". These don't drag on long and all and the solutions aren't crazy hard without being stupidly easy. You'll complete anywhere from 6-9 (roughly) of these puzzles per-playthrough.
With 6 endings and one of these 6 endings needing to be found to unlock the "True Ending", there's quite of bit of replay value. And trust me, until you've seen those two endings, you won't want to put the game down. The characters, while at the beginning seem like stereo-typical anime fare, quickly become more and more enthralling as you talk to them amongst the various routes you can take throughout the game. I'm usually the type of person who can see plot twists coming a mile awhile, but I was constantly one-upped by the excellent plot. By the end of your first playthrough, you'll begin to really care about them, and by the time you've seen a few of the endings, you'll become quite connected.
When you finish the game the first time, you're given the option to fast-track all the dialogue you've already read with the game only going to regular speed for new things (which is very useful for finding the routes in which to take for the other endings). My only issue is that you can't skip already completed puzzles. Luckily, most don't require much after you've solved them once, and in some cases you can sometimes rush right to the solution (but be careful...some of the conversations during these puzzle rooms are keys to the getting a certain ending...).
+Fantastic story and character development.
+True ending is absolutely mind blowing.
+Puzzles are very satisfying.
+Music really sets the mood, whether intense, sad, or serious.
+Lots of replay value after first-playthrough. Trust me, True Ending/Ending-you-need-for-true-ending are so worth it.
-Can't skip already completed puzzles.
-Few people know this game exists.
Get it. It is my favourite DS game this year and one of the best on the system. If you're a fan of adventure games, visual novels, or just games with great stories, this one's for you.