Just upgraded to it today. I've only had it a few hours so I can't speak to some of the things you asked about, but I did come to Android from a different OS so perhaps my thoughts can be of value.
I came from a Lumia 920 (Windows Phone) that I was actually very happy with until just this past month, when a bunch of different things happened at the same time and really annoyed me. The battery suddenly stopped keeping a decent charge, vibration stopped working, LTE data was inconsistent... anyway, it seemed to age a hundred years in a single moment. It was time to upgrade and unfortunately on AT&T there wasn't a real clear upgrade path for Windows Phone users, so I decided to give Android another shot.
That was the easy decision, though. My wife has the Galaxy S5, and using it once every few days really didn't do anything for me - I just wasn't impressed with any aspect of it. So my real targets were the HTC One M8 and the LG G3. I've spent a lot of time over the past few days reading up on both phones, reading comparisons, trying to see why people prefer one over the other, and I walked away really thinking I would go in to the AT&T store and buy an HTC One M8. Lots of people were saying that both phones were great but the M8 was just a bit faster and just a bit better etc.
I went to the AT&T store and played with the M8 and the G3 side-by-side and my opinion totally changed - the G3 really stood out as a superior device in my eyes. There was no "lag" that people keep talking about. None that I can see, anyway, although I am coming from a 20-month-old Lumia 920. But side-by-side, the G3 and the M8 seemed to be equally speedy. As far as the screen goes, both devices looked nice and I wouldn't have been able to seriously claim that one was better than the other.
Another thing that people were always bringing up when comparing the M8 and the G3 was the "premium" feel of the M8 and the plasticky feel of the G3. That all may be true, but it wasn't a big deal for me, especially since I was going to put a plasticky protective case on whichever device I bought. The G3 felt pretty nice without a case and feels pretty nice with a case - appropriately solid. Coming from a Lumia 920, which is a device that was a TRUE premium feel, with super tight fit and finish and a very pleasant heft that denotes a solid, thoughtfully built device, I was kind of worried about how I would react to the G3... but I shouldn't have been worried because it feels great.
Yet another thing that lots of folks were referencing when comparing the two devices was Sense vs LG's launcher... I just gotta say that I don't like the look of Sense. I don't like the font, the spacing of the letters. They're too narrow and too close together. Nitpicky, I know, but Sense just looked repulsive to me while LG's launcher/thing looked nice, or at least acceptable.
So I've rambled a bit but those are my thoughts after owning the G3 for a few hours and coming from a different OS.