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One of the screensavers for Amazon’s $400 Kindle electronic book reader has an image of an old printer and above it a message that reads: “Kindle is a totally new class of device. Thank you for being an early adopter. We would love to get your input at: kindle-feedback [at] amazon.com.”

First of all, while the Kindle can not be a totally new class of device (electronic-book readers have been around for a couple of years), it joins the Sony Reader in making the e-book reader class a whole lot sexier and buzzworthy. While the Sony PRS-505 ($300) is the glossier of the two devices, the Kindle is the more revolutionary in that it’s a free built-in wireless connection that lets you to tap into Amazon’s huge online bookstore from almost anywhere you are able to access Sprint’s EVDO cellular data network.

In numerous ways, the Kindle is similar to the Sony Reader. At 10.3 ounces, the Kindle weighs about an ounce more and is somewhat bigger, measuring 7.5 inches high by 5.3 inches wide by 0.7 inch deep. But both devices have 4.9-by-3.6-inch (6-inch diagonal), 600 x 800-pixel screens that use E Ink technology, which processes to make the letters and words on the screen look more printlike in their appearance–it’s quite impressive if you have not seen the technology in action before. Technically, it is an electrophoretic display, which Wikipedia describes as “an info display that forms visible images by rearranging charged pigment particles using an applied electric field.” The Kindle’s screen has 4 scales of gray and 167 pixels per inch, while the Sony’s has 8 scales of gray and 170 pixels per inch, which means the Sony offers a bit more in the way of contrast and is somewhat easier to read.

One of the inherent downsides to E Ink technology is that the screen takes a second to refresh (it goes to black and essentially blinks) when you turn a page. Some can find this “ghosting” effect bothersome, however the Kindle’s designers have done a great job limiting the delay so it Is not totally irritating and jarring. In addition, considering you may read the screen in direct sunlight (think: beach chair), the trade-off seems worth it.

To be clear, Amazon (like Sony) opted against using a backlight, since it strains the eye. That means you will need to use the Kindle in the same sort of well-lit environment that you would read a normal book or magazine. The Kindle stores around 200 books in its 185 MB of user-accessible internal memory, and you may store hundreds or even thousands more books, MP3s, Audible audio books, and other files on the device if you buy an optional SD memory card (only capacities up to 4GB are supported, as it does not accept SDHC cards). We’d have favored if the SD-card slot was not hidden behind the Kindle’s back cover, but there are worse sins. On the other hand, the battery is user-replaceable–one user-friendly feature that the vaunted iPod has not yet seemed to master.

Read more about this amazing product here: Kindle Wireless Reading Device