I'm new to microcontrollers and bought this product to start off with something easy. After a few days of trail and error I looked at the comments on the product page and realized that the firmware was kinda buggy. I also noticed that someone had written a new firmware for the backpack, so i went ahead and flashed it. This really solved everything, contrast isn't changing, backlight isn't flickering, write speed was fixed and i have seen no more strange symbols appearing when writing commands to it.
I could not find a library for it so I decided to try and write one myself with a little help from iklln6's code from the comments.
So here it is: serialGLCD library for Arduino
This firmware and library also adds more functionality like drawing bitmaps, filled boxes and 2 fonts!
As far as i can tell this works perfectly with This firmware
(Note that this firmware ONLY works with the 128x64 display!)
I have not commented the library itself, but I included an example sketch with comments
Please have a look at the library and tell me if something could have been done another way...
And Sparkfun: please fix the firmware!
EDIT: Project was moved to SourceForge