My 4GB is a Transcend 150x 4GB SD card. I originally bought the 4GB for my iMate PocketPC phone to carry eBook, business notes and some music. The internal 64k just wasn't enough for any of that data. My laptop reader wouldn't recognize the Transcend until I updated the driver but no problems with any of my other devices, including the PRS-505.
The 2GB is a generic standard speed that sells for $9 at a local computer store. It was given to me as a gift. I really didn't need it but apparently it was too cheap to resist?
It sits in my drawer as a backup but I may give it to my spouse for the new digital camera.
I'm sure there are some compatability issues still out there. Probably based on brand, size and or speed. Some higher speed SD isn't liked by older devices, or non-cameras. It's the price of technology.