There's two 'quality' factors to look out for (well, three I suppose); but the one I struggle with the most is the quality of the rendering of the ebook on the actual device in terms of layout/typesetting.
I spent a considerable amount of effort in getting manuscripts "right" for print production, alas when you convert to ePub you lose a lot of the beauty and then you also have to deal with the way the device itself renders the eBook.
There have been reports of less than stellar conversions (including spelling mistakes) which I suspect might be due to the physical book having being scanned and converted, as opposed to creating the eBook from an electronic manuscript.
Unfortunately of course, if the original manuscript was bad to start with then there's no saving it
Paul.