I had the chance to take the HTC One M9 out on a tour of La Sagrada Familia in Barcelona and brought along the M8, Sony Xperia Z3, and Apple iPhone 6 Plus. A few comparison collages are included in this gallery to give you a sense of how the HTC One M9 performed.
HTC stated that the final release camera software continues to be tweaked so it is likely that the M9 camera will be even better than what I saw a couple days ago. I noticed that there appears to be a greenish tint in the software now and am sure that the software can be improved over the next few weeks prior to release.
I also noted that the M8 photos were rather poor and later discovered there were scratches on a protective film of that lens. While the lens was wiped clean, I think these micro scratches led to fuzzy edges in the shots. Another M8 owner told me to physically scratch off that film and I now see it is removed and photos are better than what I captured at the time of this tour.
You may be able to see in these low resolution gallery images that the HTC One M9 and Apple iPhone 6 Plus appear to be the best in most comparison shots. The M8 is clearly the worst and sometimes the Xperia Z3 is best.
Picking the best depends on your personal preferences too. As you can see, the iPhone 6 Plus offers yellower softer images than the others, but some people also don't like this look.
One aspect of the M9 camera that disappoints me is the lack of OIS. The iPhone 6 Plus has OIS, but the Xperia Z3 uses digital stabilization. I will have to test out video quality when I get a review unit as that is where OIS becomes even more of an issue.
The bottom line is that the HTC One M9 camera is an improvements over the HTC One M8 in most situations. If you use a M8 for lots of low light photos, simply flip the M9 around and use the improved UltraPixel camera now found on the front. With the M9, you really do get the best of both worlds in a single smartphone and I think the One M9 is likely to be my daily driver when it launches on T-Mobile.