i have a dx. i am at the airport reading it now (long international flight, but no worries with this week-long battery). i love it and highly recommend it, but with one huge caveat.
the dx is a very expensive product with old software, old hardware, and limited capabilities. it is infuriating that amazon will neither update the software nor tell you if they even plan to do it at any time in the future. don't expect to get asian fonts, page numbers, or any number of things you get with the less expensive 6". this is a sore point for many of us in the dx community, and amazon's stance makes it pretty unlikely that i will ever purchase a tablet or other similar device, because i know they won't measure up to apple's phenomenol after purchase support. the hardware is more than a year old now, with no hope at this point of something like wifi capability. pdfs need to be formatted just right to work, and page turns with pdfs can be slow. amazon made a great device, and then dropped the ball.
but, the dx is undeniably the best large screen ereader on the market. it is a true joy to read eink on the large screen. the screen size is particularly important if you read quickly, because it displays a lot of text at once. i find my reading speed hampered by the small screen of the regular kindle.
anyhow, considering how versatile the ipad is, and how well it handles stuff like zooming (not to mention color), it is the better buy for most people. if you are like me, though, and read lots of books, dissertations, and journal articles that rely primarily on text to convey their message, then you'll at least want to consider the dx.