This is a niche product. It runs on android. That doesn't mean its usefulness has to be measured by its ability to play fruit ninja. This is NOT a gaming/internet browsing/email reading tiny computer that happens to be more expensive and slower than other tiny android/apple products. This is an audio/video player that is so high-end that it has the processing power to run android and a slew of android apps.
Those apps are not it's main selling point, nor it's main redeeming feature. They are a nifty plus that make you feel better about getting a media player instead of a fancy phone.
UPDATE: Now it does all that app/internet/gaming stuff pretty well too since it got updated to Android 2.3.
The Cowon D3 is about as good as it gets for both sound and video. The audio quality is amazing, loads of formats are supported, and it can be tweaked with BBE settings and an equalizer. The video quality is amazing too. The D3 has a beautiful amoled screen, enough supported codecs to make it very unlikely that you will have to convert a video, and it plays in full HD resolution smoothly. This makes it even less necessary to convert videos, and with a separate cable you can output the awesomeness via HDMI. The built-in internet browser doesn't play flash videos. Instead, those are opened in the JetVD program. Jetvd plays youtube HD video beautifully. I'm really impressed with its quality and ability to play nice with the browser.
The D3 has had several firmware updates, and they make a huge difference. All the professional reviews out there now for the D3 complain of its sluggishness. With the new firmware it isn't sluggish at all. It isn't silky smooth in all things like a true android tablet, but it flows smoothly through audio, video, most non-game apps, and its own interface. This isn't just low standards on my part, I tried the firmware it came with for a while (3.25) and it was painful. Lag galore and it would randomly lock up and need reset. I would have sent it back if firmware 3.31 wasn't already out, 3.31 fixed the lag and lockups (all of them, I haven't had any since upgrading). Plus, 3.33 is out in Korea. It will be on the Cowon english site in a few days, and there is no reason to believe it's the last to come.
The interface is heavily modified by Cowon. It sets the D3 apart and emphasizes the fact that it isn't primarily a tiny tablet. It works as a tiny tablet though, thats what you pay extra for over Cowons last model, J3, which is purely a media player. The D3 runs most apps that are not CPU-intensive. It's terrible with games, just terrible, but I have unit converters, alternative keyboards, app stores, calculators, fring, wifi analyzer, a to-do list widget, and a bubble level that uses the accelerometer installed. They all run flawlessly. Some of the apps were made by Cowon specifically for the D3, like the impressive music player and the snazzy clock on the home page.
This is an opinion that won't be shared by everyone, but I think the D3 looks more sleek and professional than any phone or apple device I've seen. And I find that the unique interface has the same slickness. It also feels pretty sturdy.
In a nutshell, if you are shopping for an android tablet to horse around with, this is a very pricy, sub-optimal choice.(UPDATE: now it's much better for this) You'd be more impressed by a cheaper Archos tablet or a fancy phone. But if you wan't an audio/video player that reeks of quality and can also run handy android apps and surf the internet, this is about the only option that has its priorities lined up in that order.
If you were to buy just about any other android device, you would almost certainly gain processing speed and gaming ability. You would also certainly lose sound quality, video quality, dedicated media buttons, and the unique interface. By far the closest competitor to the D3 (Archos 43) has an inferior touchscreen (resistive, not multitouch), inferior viewing screen (LCD), inferior sound quality, lacks media enhancement tech (BBE and JetVD), Inferior build solidity, a fairly generic android interface, and it looks like a toy with its prominent pasticy rounded ends. But...you can play fruit ninja with it.
UPDATE: The Cowon D3 got an update to Android 2.3 and I am happy to announce that it can now play fruit ninja perfectly. Overall, the D3 is much quicker, and is now a capable internet and horsing-around device. Now there is less of a compromise between multi-tasking fluff and supreme audio/video quality, because the D3 does both.