Since I've been posting book reviews, I've already had a few questions about the Kindle and what I think about it.
I have the 9" DX with 3G and love it. The 3G is so useful. You don't need to have a 3G provider account and it just seems to work almost everywhere I travel to (including even China). I'm not sure how Amazon make that work but it's great. I see a book, click to order it and moments later the book is on my Kindle.
The other big advantage is that it holds so many books. I've got a good library but it's at home and I'm travelling a lot. I now keep the books I use most on the Kindle instead and it's always with me. That's a huge advantage.
I find the 6" device screens too small. I'm sure I'm getting old and eyesight isn't what it used to be but technical books on 6" screens just don't work for me. A novel might be ok but not a technical book. The up-side of the 6" device is that I've seen people put them into their pockets. You won't do that with the 9" device unless you have really big pockets. It's more like an A4 sheet of paper.
I like the way that I can email a PDF to my Kindle and it's delivered through the 3G whispernet. There's a small charge but it's very useful and the options for controlling charging are great. While you can change font sizes, etc. for Kindle-format books, you can't do that for PDFs that you send to it.
The other reason I like the Kindle rather than say an iPad, is the electronic paper screen. I only charge my Kindle once every few weeks and the screen is a joy to read. It basically only draws power when you turn the page. By comparison, the iPad and other such devices have an active screen and I find them very hard on the eyes after a reasonably short time. By comparison, I can read the Kindle all day without an issue. Unlike the Kindle, active devices also need constant charging to be useful.
If you get a Kindle, you need a cover for it. I got the Belkin Neoprene cover from Amazon. It's excellent also.
The downside of the Kindle? You really will be buying your books from Amazon and not all titles are available in Kindle format. But I've found that availability is getting much better almost daily in the technical book space.
If you want to read old classic books as well, there are a *really* large number of them available in the Kindle store for free.
Overall, I couldn't imagine life without my Kindle now.