I received mine today. Initial thoughts are:
Beautiful hardware. It feels great, just the balance of size and weight. Having owned a Galaxy S1, S2 and an S3, I never thought the quality of the chassis would be important to me but it's just fantastic. I don't appear to have the issues that some have complained of either.
I'll have to adapt to the screen. It's perfectly crisp of course, but does look washed out compared to the saturation and perfect blacks of the AMOLED displays I am used to. Despite this, I guess the contract ratio will be ultimately better on the HTC One and that the colours are probably more accurate. I've yet to test it in my typical usage scenarios though.
In terms of software, I don't like the stock ROM. I don't like the apparent absence of a menu button although I understand that I can fix that with a different kernel. My main bone of contention however is that I feel like my S3 is slicker; even more so with the custom ROM I currently have on it, although that is a little unfair. One example is the game The Tribez. Scrolling around the map is choppy on the One compared to my S3 on either stock or custom ROM, which is like butter. Likewise with Chrome, and to my surprise the One scored significantly worse on Sunspider in Chrome which I was not expecting at all. With the stock browser it scores slightly better.
I'm looking forward to running something like Supernexus, Paranoid Android or CM10.1 on it where hopefully the chipset will have chance to shine. I hope they don't ruin the camera features though.