IIDX is a hard game. I often describe it to people who are only familiar with Guitar Hero, Rock Band and the like as "Everything Hero." On the hardest difficulty (called "Another," for nebulous reasons), you play every part of the song, or close. It's far more like real-time sequencing than DJ'ing, really. Here, as an example, is autoplay of a song from the home version of DJ Troopers with an uncharacteristically Japanese title called "Child's Sketchbook." The tune's originally from Drummania/Guitar Freaks, another Konami music game series. Folks refer to songs that jump from game to game like this as "transplant songs."
Man, isn't that ridiculous? Especially 0:48! This is, actually, kind of the norm for hard songs in this game. Not necessarily in terms of sheer numbers - there are still lots of easy, or at least, easier songs - but there are more memorably virtuosic songs out there with every new version. It's hard for them not to dominate your impression of the series as a whole.
Now for some attempts at providing context: IIDX has always been about catering towards the fairly small group of people that obsessively play the game in Japanese arcades, or wish they could. In the arcade, you buy a card that keeps track of your scores. The card also allows you to set other players as your rivals, comparing your scores with other players in minute detail - you can play "against" other players' scores as ghosts, like in a racing game, with a little +/- number keeping track of where you are scorewise with an opponents' best score on a song, and a score graph that lets you easily compare your performance with others and yourself. There's even a database included in each of the home versions with score data for every player registered in Konami's system from a certain date before the game's home release, giving everyone a shot at something resembling live competition.
Another concept that I feel is integral to understanding the people who play this game is that there's almost no memorization involved. Guitar Hero and Rock Band have convinced quite a few people I know that music games are all about memorizing the note patterns for the super hard songs and practicing them over and over again until muscle memory takes over. IIDX can be played like that, sure, but the vast majority of players opt to focus on honing their reaction time to a razor's edge.
There are a number of options, adjustable before and during every song, to let you do this. The first is "Hi-Speed," (adjustable from 0.5 up to 5) which spaces the notes out and makes them scroll faster. If you've played Rock Band with the "Breakneck Speed" option turned on, it's a little like that. The difference is that it's totally controllable. Hold down the start button in the middle of a song to bring up a list of the available hi-speeds (seen above). Press the top row of buttons on the controller to increase the hi-speed, and press the bottom row of buttons to lower it. It's pretty dang handy.
The other helpful feature is called "Sudden+," which causes a square pane to appear over the area where the notes appear (seen above). Hold down the start button and spin the turntable up and down to adjust how much of your view is obstructed. Combined with Hi-Speed, you can make the notes scroll at any speed you like, depending on personal preference, the song's BPM, etc. The result of all this is that you eventually learn to read notes that scream from the top of the screen at speeds that seem, initially, to be entirely unreadable.There's a second phase to all of this madness, and it's called "Random." Like Hi-Speed, it's turned on before you start a song. Do so, and the default note patterns of the song (a big reference list can be found here) get randomized. Any note that normally appears in the 3rd column gets swapped over to a random other column, and so on for every other note in the song. In the above picture, the left side player has random turned off, and the right side player has random turned on.
The result is that every song can remain surprising for many, many plays. More importantly, you get to play a huge number of different patterns that don't necessarily exist in any song by default (I'm not even going to start talking about "s-random," which randomizes every single note, as opposed to just swapping the note columns around). A common topic of conversation among beatmania players is the notion of songs having "good randoms" or "bad randoms," or being more prone to either. It's a fascinating extra layer of gameyness.
Ok! Why's the home version of IIDX 15 so especially nuts? The reason is: they added a new, harder difficulty for around 20 songs, one of which was already one of the hardest songs in the game.
Let's compare. Below is a video of Mendes, which was already the "boss song" of IIDX15. On the left side of the screen is the Another difficulty setting (previously the highest, above Hyper and Normal difficulty). It has 2000 notes in the span of 2 minutes. On the right side of the screen is the new difficulty they added for the home version, which no one's really sure what to call yet ("Danger" difficulty and "black another" are frontrunners). It has 2626 notes in the span of 2 minutes, and is the closest thing yet in this game to a challenge that is genuinely physically impossible. As of this writing, there are an estimated 5 people in the world capable of clearing it. I think it's a good place to stop at, really: