Hello.

I have used them all.
The Kindle is kind of cool, Amazon offering.
Nook is the B&N offering.
And Sony e-reader, well,,,,.
I lump them together as they are more alike than not.
The first thing about them is the e-ink, Much easier on the eyes to read than even the best CRT or LCD display.
The second thing, black and white, yep, not a drop of color.
The third thing, speed, or should we say lack of?, the screen "refresh" is real slow, but it is a static display after all.
Next is the operating system, all use some form of embedded that is not normally open to the user.
WiFi is now the norm across the board but anything even close to real browsing is not an option.
MP3 player? sort of, but hobbled by both the OS as well as the size of the unit, clearly no threat to the iPod market.
But, with all this being said, as a portable "book" it is about as ideal as we are going to get at this time.