iPad, nook, Kobo, or Kindle, which to buy!?
Ok, you've got choices!
You might be focused on the Kindle keyboard, the size of the Kobo, the color touchscreen of the nook, or the glitz of the iPad - all considerations in your decision to purchase a device for reading e-books. Another thing you should consider is the investment you are making in e-books and what store you'll use. If you plan on buying a lot of e-books you may consider making your investment in the device that matches the marketplace you want to use!
At this time the major bookstores with the most titles are B&N and Amazon. The selection from Kobo, Sony, Apple and others is just not quite there yet. While the "minor" players may have best sellers I think you'll find their collections incomplete. At face value, B&N has more titles than Amazon but it's hard to compare because they don't disclose the number of titles that are "Google Books" content (fair amount of junk). My experience is Amazon still has an edge in the e-book market but B&N has made up a lot of ground. I don't follow periodicals, but users say Amazon has B&N beat there too. The bookstore matters because the B&N and Kindle bookstores are no longer device specific. If you have a mobile phone, iPad, etc you can read all your Kindle books there. Other new eReader devices are tying themselves to these major marketplaces.
I've put together a few things I think would push me in one direction or another.
nook: Free reading in B&N stores for 1 hour a day, Lending a book one time for 14 days, use books from any source that uses ePub (Kobo, your local library, and others). B&N has an Apple, iPad, desktop, and other supported devices - but no syncing. Decent e-book selection but not as good for magazines, etc.
Kindle: Great selection and probably best prices overall, great if you plan on mostly buying eBooks (and a lot of them). Same device support as B&N but syncs your books between your readers. No support for other bookstores at all, you are locked into Amazon.
iPad: iPad has the ability to get books from many stores (Apple, Kobo, Kindle, B&N) but still no way that I'm aware of to load DRM protected ePubs from other sources like your local library (Stanza can handle non-DRM epubs). If you want to pick and choose totally based on price and availability, don't mind spending more money on the device, and want a multi-purpose device get an iPad. One warning, it's larger than the other devices and heavier. It may not be as good of a device for long reading sessions.
Kobo: Kobo has a growing online store and multiple device support. I've been frustrated with the selections in their bookstore when shopping for my nook. The Kobo has one unique feature I've not seen on other bookstores, read books on the web. If you are considering a Kobo reader or iPad and want to read books you buy with a web browser check out their offerings.
eBook readers have made publishing significantly less expensive and over time all of us bookworms will benefit from more authors and cheaper prices. If you're and author there are eBook publishing companies that can help market and publish an eBook.
I love my e-Reader, but only as I've considered all my sources for e-books did I realize what a big part of the purchase that is!