Pros: Excellent audio, good quality screen for video and pictures, ok interface with touch screen.
Cons: Not enough options, most features have potentially deal-breaking flaws.
So, you've probably already read up on this device. You likely know about the touch screen, the excellent battery life, the SD expansion slot, etc... So I'm going to write mostly about the things that I didn't know until I finally got the product. This device, while having more features than anything I know if in it's price range, also has a series of possibly deal-breaking flaws and shortcomings that undercut nearly all of it's features.
This is the player's strong point. It is widely considered to have very high audio quality, and I have to agree. It also has a decent assortment of adjustments available for tweaking the sound, including a 5 band equalizer and a number of effects provided by JetAudio, many of which are quite useful. It also supports a lot of non-standard file formats making it a very versatile player. Also, for those who don't want to use the touch screen all the time, you can set the buttons on the top of the player to pause, play, and skip through your music.
While there's nothing to really complain about quality-wise, there's plenty to complain about when it comes to the interface. While it's a convenient device for getting music onto it, being that it acts as an external hard drive and you can simply drag and drop music onto it, it has a relatively slow connection speed, and has limited playlist support. It does not read standard M3U playlist files, and playlists cannot be created on the device itself. It has to be synced using software like Windows Media Player and the playlist is created there. Also, playlists cannot be used with music stored on your SD card.
It also apparently doesn't have proper support for shuffling. I don't have a lot of experience with this myself, but from what I've read, if you regularly use the shuffle feature, you'll soon hear patterns begin to emerge, where some songs will repeat while others are completely ignored, and it also becomes somewhat predictable. How this could be a problem on a device in this price range, and after well over a year of firmware updates, is a confounding mystery.
Finally, there's the player interface itself. While it isn't too difficult to find your music, it is difficult to get to it quickly. The menu system can be a bit clunky and it can take a while to get to where you want to be. By default, when you wake the screen up to change something, it'll be exactly where you left it, which is usually at the current song playing. I'd rather it went to the Music folder, but it offers no options to customize how it works.
Then there's the external buttons. When the player is in my pocket, I like that I can set the buttons to pause, play, and skip through music. However, when you hold the forward and back buttons, it fast forwards and rewinds. I have no use for this and would rather they skipped through entire folders, but there is no way to make it do so.
The D2 supports a couple different formats of video, including XVID encoded AVI files and WMV files. Playback is smooth at 30fps and surprisingly clear and watchable on such a small screen.
Actually getting a video that will play can be a daunting task. It's unlikely that you'll find many videos that'll play as is, and usually those are WMV files. Typically, you'll have to convert them yourself using either the included software or other third party programs (currently I'm using AviDemux). Video conversion can be a complicated process for the less technical savvy, which is probably the majority of people who are shopping for an MP3 player.
When you do get a video to play, you'll likely notice that there's a slight delay in the audio, about 1/5 of a second. I don't know why it does this, but it does. If you want the sync in your video to look right, you'll have to manually un-sync the audio so it plays 1/5 of a second sooner than the video. So far, I've only been able to do this with AviDemux.
The D2 comes with a text viewer that can read standard TXT files. It has a simple interface and the text is crisp and readable, if a bit small.
Most ebooks come with some kind of formatting applied. This will cause them to appear broken up on the D2, where you'll get one full line of text followed by a couple words and then a long gap. This can make reading long texts difficult. The work-around I've found is to open the HTML version of a book in my browser, copy and paste it into notepad, save it and use that.
A more serious flaw however is the fact that it will always fill the lines from edge to edge, meaning it will split up words that don't quite fit. Often these will be words like "of" and "the". This makes reading anything longer than a few paragraphs incredibly taxing as you have to go back each time to piece the word back together. Why this hasn't been fixed is beyond me.
Then there's the font size. For some it'll be just fine, but for many it'll likely be too small. The D2 provides two options, a 1.5x font that is just a horizontally stretched out version of the normal font that, while bigger, is actually less readable. Then there's the 2x font which is very big and readable. However, in both cases, since it can fit less words in a line, it cuts a lot more words into pieces making reading even more frustrating.
Finally, there's the lack of options when it comes to the interface. You only have 5 color schemes to choose from, the default blue, and 4 that use ridiculous, uncomfortable-to-read colors. No black on gray or anything as simple as that, and no way to create your own. Also, there are no bookmarks to keep your place. If you want to return to a spot in a file, you'll have to write down the number that represents that spot in the book and input it when you return.
The sharp and bright screen makes for quite an excellent picture viewer. While it can be a bit slow to load, it does give you the options to zoom in on photos and pan around them, and view your photos as a slideshow.
Mostly minor flaws, but still annoying. When I browse pictures, what I want to do is have a full screen image, and then be able to just click on something and go to the next full screen image. This is impossible.
From the picture menu you can see a bunch of small thumbnails that represent your photos. If you double click on one it will blow up to full screen and look very nice. Then if you want to go to the next or previous photo, you tap the screen to bring up a menu which obscures the left, right, and bottom sides of the image, and then click on the corresponding arrow. The menu will remain there for 7-8 seconds before disappearing and allowing you to view the image full screen unobscured. There's no way to get the menu to go away sooner, no option, nothing to click, nothing. In the text viewer, you click the left or right side of the screen and goes to the previous or next page, only blinking a directional arrow for a split second and then immediately going to full screen. Why can't the picture viewer do this?
One alternative is to use the slideshow mode, but this has a number of drawbacks. Number one is that, by default, it breezes through the photos at about 1 second per. By the time you get into the menu and set it to slow down, you'll have to start over to look at the 5-6 photos the just flew by. It will only let you slow it down to about 5 seconds though, and the settings are only temporary. You'll have to do this every time. You can pause the slideshow, but then you have that menu that sits there again.
Finally, what I do, is just keep going back to the thumbnail screen and double clicking each photo individually. It's slow and tedious, but quicker than waiting for the menu to go away.
Later firmware revisions added Flash Lite 2 support to the player. This allows you to run various Flash games and programs on the device, and to program your own if you own Adobe Flash.
Good luck finding anything worthwhile. I've probably installed around 50 working games on the device and maybe 3 of them are somewhat worth playing. Someone programmed a couple decent mini-programs, like some calendars, a notepad and a stopwatch, but I have yet to find any other useful ones, like perhaps something to replace the nearly useless text viewer.
Also, most games are not designed to be played with a touch screen. The D2 compensates by letting you use virtual arrows, with which you simply touch the edges of the screen to choose a direction. This is of course far less intuitive than having a directional pad, and the screen will be constantly obscured by your fingers (no way you'll pull it off with the stylus if the game is even marginally fast-paced).
There are some games that do work with the touch screen interface though, but the problem with most is that when you're touching the middle of the screen, you're blocking your view of a lot of what's going on. Sometimes it's ok, but other times it makes a game more frustrating than it should be.
Finally, there's the fact that it's only Flash Lite 2. If they were to upgrade to support Flash Lite 3, it's possible the player could be given native support for FLV files, the very common file type used for all Youtube videos, which would mean playable videos for your device would be quite plentiful and easily obtained. Perhaps there are technical reasons why it is not supported, but if these could possibly be overcome, doing so would make the device far more desirable to consumers with no desire to become digital video technicians just to watch a kitten dancing on a piano.
One last thing:
The battery is rated for 52 hours of audio, and 10 hours of video. This is extremely good for any player, and from what I've seen and read it does seem to perform close to what it's rated.
Proprietary AC Adapter connection. This means you have to use the included plug, which means you won't be plugging this into your car unless you have a power inverter that allows you to plug in standard power cords, or if you somehow have a USB port handy, or if perhaps there is a car adapter for this kind of port that I'm unaware of.
Then there is of course the non-replaceable battery pack. Though it has a long life, it will diminish over time. This is true of most portable players, and is less of an issue than it is for most since at half it's life it would still beat most others, but it still means that even if the thing never breaks, it will still eventually become far less useful with a weakened battery.