The kindle is serious reading pleasure for me. I actually get a lot of reading done without getting my eyes tired. I still have the GEN3 (photo). Now you can get a Kindle Touch for $99 and a Kindle Fire for $199, however the older Kindle is just brilliant.
Why does the Kindle Rock ?
1. The screen quality
It reads like a paper book, it does not reflect in the sun. You can read great quantities without getting eye-tired. This is especially important due to the large amount of books I have on my list ;) - It is true though that the Kindle format is way better than the average pdf. Most pdf books however read well enough.
2. Device management
Navigation is pretty simple. The touch version probably is better, but the older one with the sidebuttons works quite well. The Amazon store integration is awesome. You buy a book online and it syncs automatically to your Kindle. I am not sure how 3G is offered now, but when I bought my GEN3 Kindle about a year ago, 3G was just a little bit more expensive, so ideal for travelling independency. But without 3G you can still sync your books via Wifi, or just connect your Kindle to your laptop for old-fashioned USB transfers.
3. No distractions (!)
This is an important argument for me to stick with the Kindle for reading. I am a big fan of Apple products, but when it comes to reading, all (social) apps and notifications are not doing any good: I never get much reading done. This is not the case with a Kindle: you basically use it for what it is good at: accessing ebooks. As many things: the simpler the better and the more efficient.
The design is great. I think the screen size is ideal for reading. You can also change character size, screen orientation (vertical vs. horizontal), spacing, etc. Again I have a GEN3 that had this all figured out quite well already. I imagine the newer versions only got better. The GEN3 already had MP3 support which you can easily toggle with alt-spacebar, so I can even 100% concentrate in public places. It even had text-to-voice already, so at least the Kindle formatted books can be read out loud if I want to listen for a change. Another important thing to note is the large battery duration. I don't have the specs at hand, but it is not often I have to recharge.
These are some highlights of my satisfaction with my GEN3 Kindle. Conclusion: low distraction rate, reading-friendly paper-like screen format, and easy book syncing (pdf / epub / usb / purchased). For me it's the best reading device out there.