Recall that I said "physically".

I don't miss the physical book for romantic reasons (turning a page, smell of the paper, all that stuff). I have discovered, however, that I instinctively use the progress in the book to gauge the story (which is why I hate excerpts at the end - they throw off the count). And I gauge the progress through subconscious physical cues from the book.

The slider bar is a poor substitute for the physical cue for me. If the iPad uses a visual cue of the number of pages left (I haven't tried one enough to know if this is the case) then it might be a suitable substitute.