Rockbox is an opensource firmware replacement that can run on several digital audio players. The firmware supports dozen of audio, video, image, and document formats and has many customizations with a spartan interface. I used to taut the product, but it has gotten extremely buggy over the years, so I no longer suggest it to anyone. If you are looking for a smartphone jukebox program, I suggest foobar2000 Portable.
- The firmware supports a vast array of audio formats as well as some video, image, and document formats. Far more than any default firmware.
- The firmware is nicely optimized and out-performs the default firmware for every device it supports when it comes to supporting large collections of files and audio decoding.
- There are a lot of customizable options, far more than any default firmware.
- By using a standard firmware across multiple DAPs, you don't need to re-learn the interface each time you get a new device.
- The software takes advantage of most of the features of various hardware including radio, and microphone options. It even supports voice-commands.
- Since smart phones have mostly replaced DAPs, most of the programmers have left the project so new development has slowed to a crawl. The team rarely releases stable updates, and, when they do, they're not stable.
- The developers have been pretty strict in their refusal to implement features desired by a large percentage of the community such as customizing the behavior of the buttons, handling meta tags of multiple artists according to the proper specs, including album art in formats whose specs don't support it, etc.
- For the past few years, the firmware has gotten more and more unstable, so much that I no longer endorse the product. I have two devices that routinely crash, even on the latest "stable" release of the program.
- The known bug list is huge (over 450 problems), and many of the bugs have gone unfixed for years. I personally discovered and documented a few bugs, but they too were never fixed.
- While a lot of the support administrators are helpful, there is one in particular who has been quite mean to newcomers in several instances I've witnessed. He or she is a major reason I didn't want to contribute further to the project.