I recently switched from iPhone 4s to the Galaxy S5. At no point was I ever unhappy with my iPhone. I loved it. But I saw that Samsung was doing great things in terms of multitasking, larger screens, and superior cameras in comparison to the iPhone. I felt like Apple was stuck in the mud and that the various versions of the iPhone 5 just didn't really excite me. It was the same stuff in a slightly different wrapper.
I absolutely LOVE this phone. It's not perfect, but it's pretty damn close. Having never owned an Android phone before, I could only go on what I heard and the 1 constant draw back I kept hearing was that because the Android environment is more "open" than Apple's, the phones themselves can be a little glitchy from time to time. I'm only about 3 mo into life with my Galaxy and I definitely know what they were talking about. The phone does get glitchy on occasion. But the selling points to the phone that brought me to it in the first place still far outweigh the glitch factor, so I have no regrets at all.
That said, if the latest and greatest iPhone that's on the market in 15 months when I can upgrade again has caught up to Samsung (multi-window, bigger screen, high resolution camera, etc), then I very well could be switching back. Time will tell.