Been wanting one of these for some time but the negative display put me off. I eventually got one and when it arrived as I expected I could not see the display clearly, also the backlight function is pathetic - I am surprised Casio allowed this to happen. For me I will use this at work and found it hard to read the display in my own home , so how will I see it on a building site?
Anyway I have changed the filter and it is now the watch it should have been

Bear in mind this wasn't that easy and took me a few hours but I am extremely happy with the outcome, for some reason the module went into hibernation and would not allow me to access any functions, this when I pressed the buttons nothing happened but the display was still present - this was weird and frustrating, even fitting a new non rechargeable battery didn't change anything , but as soon as I put it all back in the case and the contacts touched the solar panel it went back to fully working and showed me 2.bars on the charge level
If you are wanting to recreate this effect or simply replace the polarising filter in your digital watch be it any make then I have been in touch with Riley who runs
www.mygshock.com ad on his site he has created a fantastic guide of how to carry out this process
Here it is an thank you Riley
http://mygshock.com/wiki/How_to_convert_a_plain_DW-5600_LCD_to_a_negative_display