The readability of a page on an e-Reader is very, very good. The soft background and sharp clear letters will allow you to read longer perhaps, than you would if using a paper book, or laptop/computer.

The only drawback has to do with the page turning. Good but not great. In some readers, the page turning may be seem very slow, at least initially until you get use to it.

In most readers, you don't have to scroll to finish a page, but I agree with you scroll speed would have to be acceptable.

From what I gather, all of the readers use the same e-ink screens, so they are pretty much comparable to each other.

But to answer your questions specifically, the reading experience on e-readers is far better than on a computer online, which is the whole reason for the category.

The big difference is that page background on e-ink screens is not white or whiteish but grey. Under a good lightning conditions, the contrast between page background and letters is very good (background appears quite light-grey-ish) but if lightning is low (i.e. when reading in bed) the background may appear quite dark.

Otherwise, readability is great and letters are well-defined.