Sorry I'm late to the punch... but I have a Kindle, an iPhone, and a iPad (yes, I like gadgets). Let me preface this by saying I'm a reasonably happy Kindle user, so most of the specifics will relate to the Kindle, but many aspects apply to other eReaders. Here are my general thoughts:
Electronic Books in General
Some may question why one would want books electronically in general. Here are a few reasons.
- Usually cheaper
- No shelf space needed
- Searchable
- Quotable
- More physically portable (depending on the reader)
- Fair number of out-of-copyright books available for free
Downsides include:
- New books come with DRM (Digital Rights Management)
- New books can't be loaned/shared (because of DRM)
- Technology risk (if tied to a particular device, and device goes away, or company goes out of business, what then?)
- Not everything available (and some new releases are delayed by publishers so as not to cannibalize hardback sales). In addition, books still in copyright older than 5 years tend not to be available unless they are sufficiently popular.
E-Ink Based eReaders
eReaders the use the current generation of e-ink technology (kindle, nook, sony reader) work well for reading novels. The 200 DPI e-ink is easy on the eyes, and causes less eye strain than back-lit LCDs. Since e-ink passively maintains the current image, battery life for e-ink based eReaders is generally very good.
The current generation of e-ink does have a big downside: slow screen updating. The current generation of e-ink displays takes about a second to completely update the full screen. This makes zooming in/out or flipping quickly to find a page an extremely tedious procedure. (The kindle, for example, doesn't really support arbitrary zooming.) This makes texts with big tables and detailed pictures problematic at best. It also makes detailed maps difficult to view.
So, for the kind of text one reads cover-to-cover in order, or even for magazines with long texty articles, the e-ink based eReaders are great.
The other upside of eReaders is portability and storage. My shelves at home are completely full, and buying a new book means going through and throwing something else out (which is a tedious process at best). With an e-reader, I buy more freely, because it doesn't take up shelf space. eReaders are great for travel, because you can load them up with books, and then travel freely. eReaders that support downloading books over a cell phone network (like the Kindle does via Sprint's network), are especially great when traveling. For example, if you've run into big flight delay, and you finished your book and have nothing to read? No problem... download a new book!
iPad/iPhone
The iPad and iPhone both use backlit LCD screens. This makes zooming in/out and quickly flipping between pages possible. The iPad's large screen works pretty well for reference books and things with color illustrations. The iPhone also works fairly well, but the smaller screen makes the amount of information visible at a time significantly smaller (despite the fairly high resolution).
The iPad is better suited for reading than a netbook, since it lies flat and offers a wide viewing angle. However, I find the lower resolution (135dpi as opposed to the kindles 200dpi) and the back-lighting induce eye fatigue much more quickly. I find I much prefer to read novels on the Kindle.
The iPhone's small screen works surprisingly well for reading (higher DPI than the iPad's), but it is still a tiny screen. It works fine as an eReader in a pinch, but the small screen size does become tiresome for extended reading. I have used it on occasion when I left my kindle at home.
eBook formats
eBook readers generally support a few different file formats. They all support text and most support PDF. The Kindle uses a DRMed version of the mobi format for its Amazon books. The Nook uses the eReader format.
Some eReaders (like the Kindle) provide viewer software for multiple platforms (PC, Mac, iPhone, iPad, Android), which lets you read your book on multiple devices. This is a handy feature, but Amazon still tries to steer one towards Amazon store lock-in with the Kindle. The Kindle can display eBooks from other sources, and software exists to (illegal) strip the DRM from DRM-ed Kindle files.
So, I've been pretty happy with my Kindle, and I use the availability of cross platform readers on occasion. It's possible to get content from other sources, but Amazon definitely steers one toward their store. It's not ideal, but at this point I hesitate to buy non-electronic versions of books.
Sorry I don't have the same depth of experience with the Nook or Sony's Reader series.