Don't let the pictures of readers in action worry you to much. When text is shown and seems abnormally sparse in most cases it's just so that it can be seen well in the picture. The text resolution is excellent on small type. Well, I assume it's excellent in small type but then I don't read small type anymore. (I didn't go blind! Books just got small!)

If you are speed reading then the reader might cause problems unless you are using the "block view" method. For normal reading it seems the time between a button push, refresh and look back to the top of the page is about the same as a book.

Power? Man-o-man, there be the rub for a major difference in readers and most likely the one you will notice. Some people can read 5,000 pages and still be going strong while others have a few thousand or even less. This is defintly going to be a "Your Mileage May Vary" thing. (Two readers here: one gets about 1,000 pages and another gets about 4,000 pages on the same books.)