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The primary advantage that HTC is claiming for the "ultrapixel" camera is that it is superior in low light. I haven't used a new One myself, but many of the reviews I've read don't seem to see a huge advantage. The concept is that the pixel size is large, hence there are fewer of them, so that you get 4 "ultrapixels" on the same size sensor that also could support 13 "normal" megapixels. Here is an article with some side by side photos by the HTC One and the iPhone 4S, and it does look to me like the low light performance is superior on the HTC One.