I debated between carrying a dedicated ebook reader vs an iPhone / iPod Touch. I used my Kindle 2 for several trips, sold it when I preordered a Kindle 3 and have been using an iPod Touch in the meantime. Here are what I see as the advantages and disadvantages of a dedicated ebook reader.
Advantages of Ebook Readers vs a PDA/phone:
Battery life. If you're doing 8-day trips, the long (30 day claimed for Kindle 3) battery life of an e-ink ebook reader is a must, especially if you're also using it as a replacement for maps or a guidebook.
Screen quality. E-ink screens look great in daylight. If you're only reading at night in your tent then you might prefer a backlit PDA/phone, but for anything but the shortest trips you will have to ration usage and turn the device all the way off between uses, making you wait a minute or two for it to start up whenever you do want to use it. I read during the day and at night and by far prefer the Kindle screen to the iPod Touch's.
Screen size. I find a 6-inch screen more pleasant for reading than a 3-inch one, and more useful if you're using it for maps or a guidebook.
Disadvantages of Ebook Readers vs a PDA/phone:
Weight. Even the Opus weighs more than an iPhone 4. The Kindle 3 is the lightest ebook reader yet with a six-inch screen, but even that is almost twice the weight of an iPhone 4.
No phone. If you're going to carry a phone anyway then this just amounts to an even greater weight penalty. If not, then it amounts to a convenience penalty of not having a free (weight wise) phone with you ebook reader.
I was debating between the Kindle and the Opus and ended up going for the Kindle.
Advantages of Kindle vs Opus:
Screen size and quality. With the new higher contrast and faster refresh rate of the Kindle 3 screens, the Kindle pulls further ahead here. Again, even more of an issue if you're using the reader to double as a guidebook.
Free 3G internet. Less of an issue on the trail, but when passing through towns the Kindle's free cellular internet access is a plus, and should be even more convenient with the Kindle 3's improved web browser.
Selection of books and ease of purchasing new books. The Kindle store has the best selection and being able to purchase new books without a computer makes satisfying specific book desires fast and easy.
Quality of rendering and ease of navigating PDFs. The Kindle's controls for being able to easily read PDFs with small fonts and figures makes it possible to actually read PDFs like the free AT ThruHiker Companion (http://www.aldha.org/comp_pdf.htm).
Text-to-speech. While many people may not want to use this, I love being able to have the Kindle read aloud to me while I setup or tear down camp.
Keyboard for journaling. The Kindle's built in keyboard means that you can use it for writing notes or as a (somewhat inconvenient) journal by adding "notes" to an existing book (can be an empty book that you add for the purpose of adding journal entries to). The Kindle 3 has a microphone, so I hope that a 3rd party app will give it audio recording capabilities for audio journaling.
Disadvantages of Kindle vs Opus:
Weight. There is of course a huge weight penalty, with the Kindle 3 weighing in at 247g vs the Opus's 150g.
As far an an iPad goes, the advantages vs an iPod Touch or iPhone is its screen size, but it comes with such a huge weight penalty that I would personally not consider it a serious option. (The iPhone 4, on the other hand, is a very compelling option and does nearly everything the iPad does.)
If you do go the ebook reader route, the next question is how to store it. Standard cases weigh too much and don't provide any water protection. I kept my Kindle in a clear aLOKSAK at all times, even when reading, and wedged it in between my sleeping pad when in my pack and never had any problems.