Personally, I think there is something to be said about a combination of approaches that I would personally find appealing. Hate to use the iTunes example, but it works really well for me. I have two apple devices, my phone and my old ipod. I listen to audiobooks on both and if I sync between the two the audiobook is bookmarked to the appropriate place when I switch between them.
So what I would like is the ability to read a book on my dedicated device, and to sync that device to my computer, but also to be able to sync reader software on my iphone for example with the same book. This would transfer bookmarks between the two devices. I don't always want to carry my dedicated device with me, but if I am say waiting in the car for my wife at the shops, I will have my phone and would take the time to read a few pages (or perhaps more if certain shops are involved :-) then sync the handset to my computer on returning home. Grab the synched dedicated reader and pick up where I had finished in the car.
I think the media is the key element not the mechanism used to consume it, which is why the whole DRM thing between manufacturers is such frustration.