In my case it's entirely opposite. My ipad doesn't get nearly as much use now since I had the Kindle 4 NT then -> K4 Touch. I have a suspicion that your enthusiasm for reading books on the new Kindle Fire will soon pass
Because it doesn't matter how you tune it on a tablet, essetially you're looking at a light bulb(dim or not), to me it's just not as soothing as reading on a reflective paper like screen, and it bothers my eyes for anything more than 20-30min of reading.