I had the iPhone 3G when it first came out, upgraded to the 4 and then the 4S. So after almost 4 years of being an iPhone user I was getting bored of the UI which hasn't really changed and got tired of the closed garden approach which Apple is known for.
Now that I have the galaxy nexus I don't think I'd ever go back to the iPhone unless Apple made some drastic changes. (Which would never happen)
I bought the first iPhone when it came out in 07. It was THE best phone by far!!! I even left Verizon to get it. When the 3GS came out I upgraded to that. I was very happy with my iPhone due to its simplicity and easy to use interface. I was able to carry one device with all of my music, videos & apps etc. The Apple eco system was very convenient using my iPhone, iPad & iMac everything just clicked. But as time went on the UI became dated, the screen seemed to be shrinking as newer phones began making bigger screens. On Jan. 9th I purchased my GNex and have been extremely satisfied. I love the bigger screen, the ability to customize it and I have just about all of the same apps I had on my iPhone. The Google eco system seems to be working great for me as well. All in all, my main reasons for switching were bigger screen, more customization, different user interface and I just needed something different.