So what's the advantage in using rockbox? Does it sound better, or different? Is it faster to load and start? Does it use less battery?
It's a extremely customizable, full featured user interface that goes way beyond the abilities of the built in firmwares in just about every DAP. Sound quality - generally considered to be neutral, at least more so than the stock firmware in the DX50. It's got a good EQ section also, but I never use that so don't know how capable that is. Load and start time is about the same. Battery use - I don't know, I've never used the stock firmware to any extent. My guess is that it's about the same.
It's power comes in how you can set it up to suit your particular needs. I'm a pro musician, and do a lot of record reviews, and a lot of stuff comes and goes through my player. So having full featured file management is really nice - I can move individual files from folder to folder, rename files, delete files and/or folders. I tend to listen in the car a lot, and listen to entire albums in there entirety, but don't like having to pick the next album while I'm driving, so I have it set up to go to another random folder/album when the current one ends, something that no stock firmware can do. And the bookmarking feature is really handy - if I decide to change what I'm listening to for a while, I can still go back right where I left off in the old thing after I'm done with the new one. And in case you haven't heard it yet, it does gapless playback by default, and effortlessly, without any glitches. And it handles just about any kind of file type you can throw at it.
And it has some nice extras - like being able to choose entirely different skins, themes, or whatever you want to call them - and if you don't like any of the ones that comes with it you can customize them. And with certain units it can be voice controlled, handy for the visually impaired.
That's just a start - see rockbox.org for more details. The one big shame about the DX50 version is that the guy who developed it hasn't gone through the official channels, despite pleading by people here to do so, so it's not an officially recognized rockbox version. Yet.