I will upgrade when the new thing is substantially better, and I will take my time researching before I spend the money.
PRS-600 was my first ereader, and I loved it. So did my Mom, so I gave it to her and upgraded to the PRS-900. Then I fell in love with the enTourage eDGe and asked for a Pocket eDGe for Christmas 2010. Had to have the big one, too (for reading at home in my comfortable chair). Realized I was no longer using my Sony so my Dad got that, whether he thought he wanted it or not (he didn't, but every time I see him he is reading it!).
I am now coveting the Galaxy Note, which will partially replace my Pocket eDGe and fully replace my cellphone that I've had for 4 years. I can't imagine giving up my full-sized eDGe, but if I do the new reader will have to have Pearl eink.