I don't have a DX, but I do have both a K2i (7 months old) and an iPad (4 days old). They're just very different toys. But considering the iPad only in its role as an ebook reader...
The iPad is bigger than the K2, but maybe in the same league as the DX? I think it's thicker though, and I'm assuming heavier. I read my K2 on the train in one hand, but I'd never pull out the iPad unless I had found a seat, and even then it'd feel like, "Behold! Make room, for I use an iPad! iPad, I say!" But again, maybe the DX isn't so much different in size. And maybe you're not reading while dangling from a strap.
Battery life is the big thing with an active display like the iPad. It's so easy to get used to the Kindle's long battery life that it makes anything else feel even shorter than it is. It's still new to me, but I'm always going, "OMG my iPad's battery is already at 89%!"
The active display is very different from the Kindle's e-ink. Personally I read the entire Twilight series in Stanza on my iPod Touch
without noticing any eyestrain, but it apparently bothers some people. The Kindle is great in sunlight, the iPad is great in Starbucks.
I've been scanning a lot of my old books for conversion to PDF, and tried displaying them on the Kindle, but found the PDF handling to be disappointing. (IOW these are image-based docs, not PDFs generated from text-based data.) Somehow the grays come out way too light, with fonts breaking up and fine curves looking blocky because the gradated edges were below some display threshold. Of course they look great on the computer and iPad.
Annotating and such is much more doable on the iPad because of the touch screen. I'm still experimenting there, looking for the best apps, so I don't know much yet.
I'm still learning what my iPad is really for, so it'll be a bit before it and the Kindle find their respective places in my life. I wouldn't give up the K2i for the iPad, but I'm not sure what I'd do if I had a DX.