Women keep telling me that 6" isn't as large as men seem to think it is. I don't find the screen too large. It's intended to be close to the size of the printed text of a paperback book but with margins it is smaller. I measured the text on a paperback and it was about 6.5".
It's a personal preference between portability and comfortable reading. It's too big to fit in a jacket pocket but it's portable enough to carry with me in most instances.
I disagree that you need both. I thought that I would continue to read on my PDA during times when I don't have my Cybook but in approx. 10 months I've managed to make it half way through Aesop's Fables. I'm not the world's fastest reader but I'm faster then that.
My experience is that you can read e-ink in the same lighting conditions as a paper book so reading with a book lamp is not a problem.
My opinion is that if you only read in 30 to 60 minute sessions then the PDA is sufficient. If you want to be able to read longer then the e-ink is well worth the investment. It's been able to replace paper books for myself and my PDA was not able to do that.