I got Rockbox on my X3 and it works very well! A nice alternative to my aging Sansa Clip+ seeing as the new clips probably will never have Rockbox running on them!
I ran into some difficulties so I figured I'd share my experience;
I first downloaded everything mentioned on the
http://xvtx.ru/rockbox/manual.htm page and started with the "first time installation" instructions. I got through steps 1-5 but whenever I'd click "upgrade" in the settings it would always say: firmware not found. I tried reformatting the MicroSD several times and tried slightly different folder structures from the instructions with no luck. I eventually set the files back in as the instructions said and looked further down the page to "first time manual installation"
Steps 1-9 went perfectly fine but I wasnt seeing progress on the Ingeinc burn tool. I figured initially that it must have been because I ran the BAT file before I installed the JZ4760 USB drivers so I closed the burn tool and ran the BAT file as an administrator and got a "Windows cannot find 'USBBurnTool.exe" error and the command promp window simply said "'inifile.exe' is not recognized as an interal or external command, operable program or batch file". Okay, so I dont want to run this administrator! So, I unplugged the X3 and started again from step 6. Once I reconnected the X3 in the fashion the instructions say, I started to get a progress bar on device 2 only in the Ingeinc burn tool window. After about 30 seconds it stopped at around 40% and gave an error saying "unable to write INI". When I clicked OK everything closed.
The scary part is that my device was acting like it was bricked after this. The power button did nothing. Plugging it into USB or a power adapter had no observable effect. Holding down options as I plugged it into a USB slot had windows pop up with: Unrecognized device. Not good! I tried disconnecting the battery and letting the device sit for a while with no luck. I removed the micro SD card; no difference. Finally I tried pressing the reset switch with a pin on the X3 and held it for around 5 seconds and suddenly it popped up with rockbox! Okay, not an ideal process but I'll run with it!
Been using it for just today so far and it works just as well as my Clip+ although it sounds a bit better. They may want to recheck the english instructions and perhaps the usb burn tool!