I've had my Kindle over a month now and I love it. It's not the be all and end all, but it serves many of my needs well.
Physical/electronic characteristics & other numbers - about the height & width of most computer books, but less than an inch thick. Weighs about 10 oz. If you leave the wireless update off and don't go crazy pushing buttons, you can get 6+ hours reading on a charge; additional batteries about $15. Built in memory holds about 200 books/periodicals, standard SD cards increase that dramatically, but you can always temporarily delete books you purchased from Amazon & re-load later. There are approx. 115K titles available from Amazon, plus you can download any other .txt, .mobi, (some) .pdf and other formats subject to digital rights, etc. This expands to potentially millions of books - many for free from Guttenberg, etc. NYT best sellers from Amazon are typically $10, other books are from $1 to very expense (I think there's a obscure technical reference for over $1K in Kindle format). Most books from Amazon have a sample for free - first 50 or so pages. Most programming books are about $4 less in Kindle than Amazon's listed discounted price - a few are actually slightly higher for reasons I can't fathom.
The e-ink screen looks like a printed page - it's not a computer screen. I can't tell you how much more convenient it is reading the Kindle version of a newspaper at breakfast (or anywhere else) vs. the printed copy.
I have about 15 purchased books on my Kindle - mostly .NET (my specialty) although a few general books - I finally got around to reading The Art of War - set me back $2; I read it while "accomplying" my wife shopping last weekend. I have another 20 books as samples - I've already decided not to purchase some books based on reviewing the samples. I also get the NYT delivered daily. I turn the wireless on for about 2 minutes in the am to pickup recent orders and the NYT - doesn't seem to impact battery life.
The personal plusses for me:
1) It's a convenient size - it's with me most of the time except when we're out socially - I always have something useful to read when my wife decides -oh I forgot to get ___ at the store it'll just take a minute - walk in with me cause its after dark...
2) The free preview for most titles.
3) If I think I might need a reference and it's available in Kindle I don't have to order it overnight from Amazon - I can review the sample and wait if/when I really need it.
4) When out & about, at a clients, or travelling I have a good bit of my library available.
5) I don't need to worry as much about shelf space for books.
6) The "environmental" issues listed below which are important to me
7) Potential savings
1) Not everything is available in Kindle
2) Kindle currently is B&W only and the e-ink as implemented looks nice for normal text, but does not magnify well.
3) Many screen shots and other graphics are hard to read, some impossible even with magnification. I've contacted kindle on this recommending they allow us to "click" on an image and expand it to full size in landscape - would sovle 90% of that.
4) To fragile & small to use as a door stop, shelf spacer, etc.
5) You can only share Amazon books with other kindles ON YOUR ACCOUNT - 6 maximum.
The environmental/financial issues:
No trees were cut down, no ink chemicals used, no power for printing presses, packaging materials, no shipping, no receiving, no warehousing, no physical inventory (including building space, inventory tax and insurance, etc.). No guessing how many to print, dealing with over/under stock, no unsold physical merchandise.
The environmental impact of the physical kindle plus the power to send content and read isn't substantially different than a cell phone or Wii-type game.
It seems to me that publishers/distributors/sellers are bypassing essentially all of their expenses except keeping their doors open and paying royalties/salaries by going to e-books. I would think that the issues with paper, ink, production runs, shipping, overstocks, warehousing, etc. add at least a couple bucks to the price of a book.
I don't expect ebooks - kindle or otherwise to eliminate physical books, but it seems a logical choice for many. My expectation is in 3-5 years 20% or more of Amazon's book sales will be in kindle format - that is substantial.