I have been using my Gen3 for almost 3 years now. I have the model with 200 MHz processor and 512 MB of ram.
What I have noticed is that when reading in the cold (eg a train platform with 5 degrees Celsius) the page flipping is way delayed (up to more than 5 seconds for just one page flip). After I am in the train and the temperature rises, the ebook returns to its normal operation.
I have not come across any permanent problem with the screen. I use the font type/size settings I settled after the first week I purchased the device and it seems like the first day (of course if there has been some constant very slow degradation I cannot see it).