Pretty much the same experience though I've also some Class 4 which seem to work fine and also some smaller (128MB - 2GB) without class numbers. I use the 128MB for PRS+ installation and pretty much stick with the 8-16GB for books... and couldn't agree more with the Sony Memory stick comment...
Originally Posted by Richey79
Class 6 is a very good speed, and the best compromise price/speed in my opinion.
You could go for class 10, but the prices go up considerably, and let's just say that class 6 is more than fast enough to handle recording HD video in a camera, so is perfectly adequate for an ereader.
I've briefly tried a class 2 in my PRS600. To be honest, In comparison with the class 6 I'm using at the moment, I can't really say I noticed any difference at all.
Memory Stick duo is the exact same product, but a different shape: you'll never be able to use it with a non-Sony device and you'd be rewarding them for a bizarre and unnecessary corporate money-grab.