I got The One on Friday as a replacement for my Icon.
The HTC One is slimmer and a lot better looking than the Icon.
As far as performance, with WP 8.1 on both, there is not really a distinguishable difference.
Having the SD slot is probably the number one reason I jumped from the Nokia (added a 128GB immediately).
The soft keys (charm bar?) is a plus, but do take a bit to get used to.
One thing I already miss from The Icon is the dedicated camera button.
Although the camera launch was slow, it was nice to be able to launch from sleep mode/lock screen.
This leads me to what I really want to talk about, the cameras.
The camera, which is the big sticking point from what I have seen, is not as huge of a difference as it is made to be.
The HTC camera app for one, is significantly better than Nokia's app, and you need to use that to take advantage of the refocus and 3D features.
The camera is overall faster at taking pictures, press to shot, but the focus is a tad bit slower.
Once you get the image, in focus, they come out sharp and color accurate.
The exposure seems to be about 1/3 of a stop dark, but I like my pictures a tad dark, so it is not a problem for me.
In the HTC (as well as the Windows) camera app, you can adjust the exposure compensation, so it is an easy fix.
My biggest complaint or missing feature, is being able to crop the image while maintain a fairly usable image.
There is no contest, with eth 20MP shooter on the Icon, I could do a 50% crop and still have a clean, usable image.
With the HTC there is little room for enlarging the image (aka digital zooming).
Although, with the larger pixels on the HTC camera, low light performance is better.
While you may see people mention that the images are just as dark, where the HTC One shines is getting better details in the dark settings, with less noise.
All in all, The HTC One and Icon perform nearly the same. The biggest difference is form factor, the camera and the external speakers.