The main differences are:
1. Firmware is a Factory Firmware Release version not a Beta
2. Reformats the HDD and updates the way it is addressed (hence the speed increase that is noticed by most ppl)
3. Better support for SDHC (now properly supports 32GB before some people had problems over 16GB cards)
4. Fixed Regensburg Syndrome. Will change back to your location within a couple of seconds (will depend on how good your antenna is and location but everyone who i have done the Upgrade for says it changes within about 2 secs.)
5. Fixed Battery drain issue
6. Correct support for MFSW
7. Correct support for Voice Control (it is hit an miss on 266X) It's now an option in the Setup menu & Voice Tag option is also an option when saving addresses
8. Fixed MP3 issue for HDD playback
9. Better response to touchscreen input
10. Different graphical displays for HDD capacity and copying to HDD
They are the main ones that i can remember off the top of my head.
You will notice when you first start it up after flashing it's all in german (language etc) however it is very easy to correct and put into english. After that even if battery is disconnected and left off overnight it starts up in english.
Carl