Death Label, developed by those Tetris Grand Masters over at Arika, is the most glorified boss rush ever made. Instead of a set number of stages like in the arcade game, the DS version consists of different courses, each one sending you up against different bosses from the arcade version of the game in a row. The play mechanics have been reappropriated a little bit for the small screens, but the end result still requires you to stay as close to the boss as long as you can without getting killed.
Interestingly, you also get a ton of points for carefully using up all of your bombs at each boss encounter, turning all of the bullets onscreen into big multiplier point items with each use. That's a scoring mechanic that doesn't show up very much in these kinds of games, which usually reward you for bombing as little as possible. Unfortunately, it also means that in a lot of cases you end up spending most of your time shooting the boss until near-dead, then waiting around until a particularly big bullet pattern comes out, bombing, and then waiting around some more.
Besides the steadily harder set of courses, the game also has an Extra Mode which replicates the fifth stage of the arcade game on the DS with a pretty absurd level of difficulty. There's also Doom Mode, in which you fight the true last boss of the arcade game over and over, with the difficulty ramping up each time.
The game has an art gallery that unlocks as you complete the equivalent of achievements ("destroy this boss," "destroy that boss," "don't bomb," etc.), which in turn unlocks a bizarre little side mode called "Oshiete IKD-san!" in which Cave head-honcho Tsuneki Ikeda instructs various characters from Ketsui itself on the game's play mechanics. There are also mini-superplay videos and assorted meta-weirdness from the development staff of the DS game. Most of it is beyond my rudimentary Japanese ability, but it, along with the included superplay DVD of the arcade version, show that Arika wasn't willing to just push this thing out the door. Sorta. It feels apologetic, like "hey, sorry we couldn't make a real game for the DS. Here's a bunch of unnecessary stuff for you to peruse as compensation." That's the kind of vibe I get, anyway.
Second opinion: here's a video review of the game by shmups forum member Rob, who lives in the great state of Alaska. You may find that he is pretty picky about these kinds of games (and maybe a little insane), but he is an important voice, nonetheless.