After progressing from cheap film cameras to digital point-and-shoots and not much liking them, something induced me a few years ago to try a real digital camera, and I dipped my toe in the water via a Nikon D3100. I went from being bowled over by its superiority over what I had used before to being conscious of how it could be better in terms of image quality, and after its autofocus mechanism broke a mere four months after I bought it I started investigating other cameras. Much research let me to a Pentax K-5, in part because it seemed to do best in low light/high ISO among crop sensor DSLRS (as you may know, it has much the same sensor as a Nikon D7000). It seemed a definite step up (in price, too!).
But as with the Nikon D3100 I started to think, well, good though this is, it could be even better and decided to see for myself what all the fuss was about with FF and rented one. Not sure why I chose a 5DII to rent rather than a Nikon, but when it showed up I was impressed by the improvement (especially low light/high ISO) in picture quality and amazed by the superiority of the L lens I rented simply as a piece of machinery. So, I sold all my Pentax and Nikon gear and switched to Canon, buying a 5DII with a Rebel for back-up (the 5DIII was not yet available). When the Nikon D600 was so cheap before Christmas I checked back in with Nikon by renting one along with a 6D and 5DIII, but the superiority of D600's sensor was lost on me; good though it is, I never preferred the images I took with it (plus, as with the D3100, I hate the convoluted controls). Canon has more lenses that appeal to me too. So for now I'm sticking with Canon (am impatiently awaiting the arrival of a 6D on Monday).