I own a DR800SG and a Kindle 2, not a DX. However, I also have an eDGe with the same size e-ink screen as the DX so I can kind of extrapolate (except for "look & feel"). I think the iRex is a better choice unless one of the following things rise to enough importance:
- Amazon Store - still the widest selection of ebooks and the easiest to browse and find what you're looking for. One-click purchasing: I like that you can download a free sample of an ebook and when you get to the end there's a link to purchase. Click and you can pick up where you left off in the full ebook in a minute or so.
- Full-page PDFs - the DR800, with the firmware update, handles PDFs better than the Kindle DX. But there's no substitute for size if you want to see an entire PDF page at once. I always switch to my eDGe for PDF viewing.
- Dictionary - the iRex doesn't have one.
Otherwise I can see only aesthetics or perhaps a preference for the DX user interface (I know I like my Kindle 2 page-turn buttons a lot) as reasons not to get the DR800.