The HTC One M8 only has four million pixels, but the device uses "Ultrapixels" which are larger and designed to capture more light. Unusually, the front-facing camera has more pixels (five million), but these aren't ultrapixels.
With a home screen swipe up mechanism, the app launches pretty quickly and you're ready to shoot. You can use the volume button to take a shot, but as these are placed in the middle of the device, it's hard to do this comfortably without obscuring the lens and/or the rest of your hand obscuring the screen, making it difficult to compose the image in the first place.
The One M8 offers a lot in terms of camera control. Pressing a set of three dots in the bottom left hand corner gives you access to controls for white balance, exposure compensation, ISO and so on. You can leave everything on auto, but it's nice to have this level of control in a native camera app.
Autofocusing speeds are generally pretty quick, but there were a couple of instances during my testing when the phone suggested that the focus had been acquired when it clearly hadn't. For macro focusing though, the HTC One M8 performs very well. In my test here it locked onto the berries first time and produced a sharp image.
Those light capturing pixels work well in low light to produce images that more than compete with the output from sensors with more pixels. Images are relatively detailed, so could even stand up to being printed.
The digital zoom produces images that are usable at very small sharing sizes, but any serious scrutiny shows up loss of detail, some blurring and even chromatic aberration.