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The x5 also comes in a extended battery life version called the x5L
MOST IMPORTANTLY: As with almost all Cowon models, this one has an excellent audio output stage and sounds very musical.
Excellent format support, including the all important FLAC.
Unit shipped with Jet Audio software for encoding, ripping and transcoding.
Modding this with RockBox removes the 10,000 song limit and adds better file browsing support.
Video must be transcoded before being added to the unit.
Included software is windows only, the unit is recognized as an external drive on Mac and Linux.
Micro USB port on the unit itself is USB host only.
USB, line in and power jacks are on the "sub-pack" which is included.
Does not support progressive JPEG
Joystick control takes some getting used to.
Side mounted headphone jack.
The 60 Gb model can be modded to use a much larger hard drive.

Here is another review posted by eBay user "kamavalli"
Features:
-What I love most about my X5 is that I can make it work exactly the way *I* want it to. I can choose my own wallpaper, I can choose how fast the FFWD scans (both through a song and through lists of files), I can map specific actions to certain buttons, I can completely customize the information I want on my screen during playback, I can control the player's power saving options, etc...
-Another favorite feature is its true universality. Linux, Windows, Mac, whatever - just plug the player into your computer with a totally standard NON PROPRIETARY USB cable, and that's it! DMP's *are* external hard drives, and there's no reason they shouldn't be treated as such by your computer, or require special software to access the files. Also, this means that you can store any kind of file on your X5, not just those which can be played by the device itself. Applications, address books, system backups, silly powerpoint nonsense...
-You can bookmark your favorite songs as well as bookmark individual points within songs.
-X5 can sort your files either according to id3 tags OR according to your own File/Folder tree-style navigational system. Personally, I despise id3 tags, and I have my own folder-based system for keeping my files organized. With X5, I can copy my files to my player and keep them exactly as they're structured on my computer, and never have to learn a new way of navigating.
-Superb audio quality. Really. Plus, a huge amount of optional audio tweaks (mp3 enhancers/bass boosters/surround sound/etc.) to pick and chose from. There's an equalizer that you can tweak to your heart's content, as well as select from the available presets.
-Supports a unusually large amount of file formats, including OGG/FLAC/TXT/JPG/AVI and many more...
-Supports lyric display during song playback
-Great battery life
-Sexy, sturdy construction. Metal, not plastic. Doesn't get greasy smudges or fingerprints. Withstands abuse!
-USB On-The-Go. Allows you to transfer files from compatible UMS (Universal Mass Storage - most digital cameras, etc.) devices without using a computer. If you find yourself constantly filling up your camera's memory card well before you want to stop taking pictures, this will spare you the pain of deleting innocent photographs to make room for the ones you haven't taken yet... additionally, in USB OTG mode, you can delete files directly from your device!
-FM radio. And you can record directly from your station onto your player.
-In addition to the standard headphone jack, you get a dedicated line-in and a dedicated line-out, which allow for high-quality recordings made to the X5 (even directly from cd's), and high-quality audio output to external speakers. There's even a built-in microphone!
-Even though the color screen is pretty small and the resolution isn't noteworthy, video playback is surprisingly good! Audio doesn't lag behind video, playback isn't jerky/grainy... it's sharp, clear, vibrant - just really tiny :)
-Built-in clock! You can tell it when to turn itself on! Set alarms, schedule recording sessions...