I have an iPad, iPhone, Kindle (b&w keyboard model), Kindle Fire, Nook Simple Touch, and Nook Tablet (like the Nook Color but upgraded specs) here that we use for testing our e-books.
From purely an e-book production standpoint, Nook is much harder to get ePub books formatted to look nice. It simply ignores many of the formatting commands so it makes it much harder. The end result is that books will often not look quite as nice on the Nook as other readers because it's too much hassle or not possible to do what we want.
Other than that I find the Kindle Fire and Nook Tablet to be very similar in capability, speed, readability, size, and weight. The Nook is a little less chunky to hold because of the beveled edges.
After using all of these (and other tablets), I would say that most people would be happy with either Color Nook model or Kindle Fire, as long as they have not used an iPad. If you've spent much time with an iPad then the others will drive you crazy because they all seem clunky, unituitive, unresponsive, and harder on the eyes. There really is that much difference in day to day use.
Apple builds premium quality products with really great software. There is no other company making tablets with the build quality, consistent operating system, or such attention to detail. Of course, the reality is that this comes with a higher price and you have to decide how much that is worth to you. However, since Apple dropped the price of the entry level iPad 2 to $399 there is no other large screen tablet even close to the features for the price. (Not considering if you can find a special deal on a refurb or something. Apple even has some 2nd gen iPad refurbs for $349 right now.) The other companies were having a hard time competing at the higher price and the price drop has made it nearly impossible.