The fact that the Korg DS-10 exists in the first place is testament to something. I don't know what that something is, exactly. But while music software on game consoles is hardly new--LSDJ, the NES MIDI cart, C64 SIDs, and Mario Paint all spring to mind--the DS-10 is the first program that I'm aware of that A) has the stamp of an actual music technology company, B) is not wrapped in a game or "art project" of some kind, and C) requires no greymarket hardware or hacking to work. That makes it kind of special, to my mind.
The danger in evaluating these kinds of unexpected niche products is the sharp whiplash of expectations: it's too easy to get carried away by the novelty of it all--or conversely, to be upset that it isn't the second coming. The truth, as always, is in the middle there somewhere. Once you figure out what it's not trying to do, there's a lot to be excited about.
The most impressive part of the cart, by far, is the synthesizer package. Each sequencer part gets two monophonic synths modeled on the Korg MS-10, plus four drum voices (these are programmed the same way as the primary voices, but they get "frozen" into samples before playback). Both synths are virtual analog units with two oscillators (each with triangle, saw, square, and noise waveforms), an envelope generator, filter (with low/high/bandpass modes), and an impressive modulation patchbay with its own LFO. (If that's gibberish to you, the DS-10 homepage has an impressive set of synth tutorials that you can watch.) Each synth can be fully automated in the step sequencer, and you can play them live using either an onscreen keyboard or a Kaoss pad interface (for the world's cheapest Theremin).
It had been a while since I'd messed with an analog-style synth, and I'd forgotten how much fun it is. Everything reacts in real-time as you twist knobs and flip switches using the stylus, and the interface has a lot of well-considered design choices. A particularly nice touch is the modulation section, which lets you stretch little yellow cords between the various input/output jacks of the patch bay. I'm not really a discerning synth tone maven, but the sounds seemed perfectly workable to me. You're not going to fool anyone into thinking you've got a Moog in your pocket, but it's hardly an NES, either. If there's anything I wish they'd added, it would be the ability to play the synths using the hardware buttons, maybe with a Band Brothers-style control scheme. As it is, the L and R triggers swap between the top and bottom screens, the d-pad moves around the signal path, and the X button is a play-pause control. That seems kind of like a waste, particularly since the sequencing itself is limited in frustrating ways.
It's not the style of it that bothers me--I like both step sequencers and trackers--it's the way that it's structured. Here's how it works: at the top level, a Song is built out of 16 Parts (capitalization Korg's). Each Part is a setup containing the settings for all six synth voices, plus a sequence of up to 16 steps for each voice. You can copy Parts from one slot to another, and you can save and load your synth/drum voices between Parts. What you can't do is control any automation across Parts, or separate the instrument sequences from each other. If you want to combine the drum pattern from one Part with the synth pattern from another, you're going to have to copy one Part to a new slot, then manually recreate the pieces from the other--there's no mix-and-match ability here. In that light, those 16 Part slots start to look pretty thin, particularly if you want a melody/chord progression that's longer than 16 steps long. Also, you'd better have your patches all set before you start sequencing--even if they're loaded from the same synth patch, changes in the voices of one Part don't apply to the others. Tweak that string sound in one, and you'll have to manually copy the change to every other Part.
But to hold this against the DS-10 is almost certainly a mistake. This isn't really a composition package like Reason. It's a groovebox, powered by a pretty decent virtual analog synth sim. And while you could probably write a song on it somehow, I wouldn't recommend it, anymore than I would recommend trying to compose on an 808. What you could do, and easily, is use the DS-10 as accompaniment to fill out live instrumentation (see: the recent Yeah Yeah Yeahs performances), or as accompaniment while writing songs on a less-restricted instrument. In a niche like that, it performs admirably--in fact, it arguably punches far above its weight range as far as cost and ease-of-use. It's a genuine musical tool for less than $40, running on cheap, durable, battery-powered hardware. What's not to love about that?