Good Read on the Ultrapixel on the One

Yeah, it's been said many times already. HTC has been bragging about the "Ultrapixel" technology since the M7, saying more pixels instead of more megapixels is better. I understand their logic, and the Ultrapixel camera isn't that bad, just for a smartphone, more megapixels (at least 12MP by now) is better. On a DSLR, not as much.
 
One thing is for sure, it does NOT act, look, or work like a 4MP camera.

This is the older version from the M7 (which isn't quite as good as the M8) vs. the S4, which if you recall is 13MP. It wins in some areas, loses in others. Not bad, vs. a camera that has more than 3x the megapixels though.

Megapixels and Ultrapixels: The Galaxy S4 versus HTC One camera shootout | Android Central

Ultimately, both the HTC One and Galaxy S4 are great cameraphones, but for different reasons. Which camera is the best fit for you depends on the kind of shots you plan on taking.

This is an accurate quote that doesn't suffer the bias of some members here.