For me, it's reading and annotating pdfs. I go through a lot of PDFs at work (I'm a researcher) and the ability to easily manage my pdfs and use a range of annotation tools really helps my workflow.
I also use my iPad for reading news through my RSS feeds, and for reading articles I've saved to Readability or InstaPaper.
For reading recreational books, I prefer my e-ink devices, although I also have Marvin installed in case I only have my iPad around.
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