I love the nook (original WiFi). I went with the nook for various reasons, but the dealbreaker was the e-ink screen (which the Kindle also has), and it's support of the epub and PDF formats.
I can buy epubs or PDFs from virtually any store that sells ebooks. The Kindle only supports a propriety format made only by Amazon. I think there may be 3rd party ways to convert Amazon format to epub and vice versa, but I haven't looked into it at great length.
The nook and Kindle both sport an e-ink screen. Words cannot describe how awesome it is. It's like magic when you first see it. Looking at an LCD screen like on the iPad or the nook color, makes me feel like I'm working, and it's difficult for me to get into "leisure mode".
If LCD screens don't bother you, there is the nook color. I got to play with one briefly today and was pleasantly surprised; it's a really nice machine. I was shocked at how good it was considering the price. And it will only get better once B&N open up their app store. The color LCD really shines with web browsing and magazines.
The iPad often comes up when discussing e-readers, but I don't really consider it an e-reader. The iPad is a really powerful tablet computing device, and the price reflects that. If you have the money to spare, and you don't mind LCD screens, the iPad has it's own iBooks app, Kindle app, and nook app. You get the best of all worlds here, plus the added bonus of everything else the iPad has to offer. It's a bit heavy for extended reading for my taste, though.