I have a Kindel and have been very pleased with it.
As one who sits behind a computer screen all day, the last thing I want to do in my free time is stare at a back lit LCD screen. The e-paper display makes for a remarkably book like experience. Further, because it only uses energy when the page refreshes, if you have the wireless turned off one battery charge will last for weeks. Plus, its great to be able to carry your entire library with you in less space than it takes to carry one traditional paperback. Also, having instant access to the book store is nice if you should find yourself stranded somewhere and wanting some new material to pass the time.
On the down side, unless a book comes with pre-defined book marks or hyperlinks from the table of contents to each individual chapter, it is difficult to just directly navigate to a particular chapter, and since page numbers differ depending on what size font you use it is also not possible to just turn to a particular page. Of course, you do have the option of setting your own bookmarks and then turning directly to them, which helps if you are using the book for reference. You can also search the book for key words and phrases, which you can’t do with a regular hard copy.
Don’t know much about the nook, but had somebody tell me that they had one and were going to switch to the kindle because it was easier to use (don’t have any details on her complaints)
Anyway, overall I am quite happy with the Kindle.