iAudio X5L: i've never been a fan of carrying around an external amp, so i've always been big on headphone amp. the X5L was easily the best sounding DAP i've ever used from its headphone out. i really wish cowon would make a successor.
Custom 5-band parametric EQ. I enjoy the ability to customize the center frequency and width of each of the bands, plus the ability to save multiple custom presets. The vibez lets you name and save as many as you want. And you have to try pretty hard to create distortion as the vibez auto-levels the total gain to avoid it.
Full control over running playlist. Being able to add music from my library anywhere I want to in the running playlist, remove music from the running playlist, and rearrange tracks within the running playlist. Plus the ability to save it as a named playlist using whatever name I want.
Ability to create/edit/delete playlists.
RioDJ style dynamic playlists.
USB 2.0 for transfer & charging. Maybe I'm in the minority here, but having it charge by USB also means zero dependency on a proprietary charger. I'll never have a hard time finding a replacement USB wall charger or car charger.
Easy access to all available file information. Title, artist, album, original artist, genre, year, codec, bitrate, file size, last modified, date loaded, date last played, play count, mood, rating, comments, composer, PCM frequency, file path.
Browse by both ID3 tags and folder. I appreciate while some prefer folder browsing. But, I much prefer browsing by tags. Browsable tags: Artist, Album, Track, Genre, Year, Composer, Mood, Cover Versions, Spoken Word (new in beta fw), album art (new in beta fw).
Dedicated volume buttons.
Single handed operation.
Both MSC & MTP support. With the production 1.04 firmware this is configurable in Windows. With beta firmware, this is configurable on the device. Many many people swear by MSC and avoid MTP like the plague, but I've grown to appreciate the ability to sync playcounts, last played dates and ratings from the vibez back to the PC using MTP.
Easy to use, highly efficient navigation wheel. Some complain that it's hard to use. I disagree as it took me all of about 5 minutes to get used to it, and its efficiency is at least on par with the iPod clickwheel.
User-replacable Li-ion battery. All you need is a paper-clip.
Available in-line remote.
Firmware/Codebase. The vibez codebase is a direct descendant of the empeg, Rio Car, and Rio Karma; and it's being actively developed to fix bugs and add features incorporating user feedback.