So I've had it for a couple of days now. Initial thoughts:
-I'm on AT&T and have yet to get LTE. As I understand, it supports 2/3 bands that AT&T broadcasts LTE, but that the one it's missing (700 MHz) is the most common, so I'm probably not the only one who would have this issue. However, I've been planning to switch to T-Mobile anyway, so that's not a big deal.
-The size and build are very, very good. I love the size and weight of it, it's easy to use one handed, and feels very premium / well put together. Haven't tested the waterproofness yet (who drops their new $550 phone in water), but it's good to know it has it. That said, the flaps are annoying. I'm going to look into the magnetic chargers, it's annoying to have to buy them, but they're only $6 on Amazon.
-Performance is great. Even running Sony's stock build, it outperforms my Nexus 7 (2013) by a long shot. This is the first Android phone I've ever owned that doesn't have any noticeable stutter or lag - although sometimes that comes with time. We'll see, but so far so good.
-Screen quality isn't bad, isn't great. I love the size of the screen, and it's very sharp, but colors are a bit washed out. It's not as bad as the Z1 - no issues with viewing angles - but it would be nice if the colors were a bit more vibrant. Still, it's pleasing to look at.
-Camera is disappointing. Most of my shots so far have been indoors, and while the color reproduction is very accurate, the pictures are very noisy. My Lumia 920 takes much better pictures. This seems to be a post-processing issue, so hopefully Sony releases some sort of an update to improve things here. I have a Sony point and shoot that doesn't have this problem, so clearly it's something they understand.... but, that said, it takes better pictures than any non-Lumia phone I've owned.
-Battery life is so so. It's always hard to tell right after I get a phone, because I use it a lot. I'll have a better idea over time, but as of now, I would say better than my HTC One was, worse than the Lumia 920. There's some Sony process (com.sonyericsson.android.wakeup) that wakes the phone from sleep over 1,000 times per hour, can't be helping things, and it can't be disabled. Maybe a root / custom rom would be better.
-The Sony skin isn't that annoying. They subtly changed pretty much everything, but unlike Sense or Touchwhiz, the spirit is still the same as stock. I'm not overly concerned with it, although I'm sure I won't use any of Sony's add on apps.
I think that's all I've got for now. So far, I like it - a lot better than I did the HTC One Mini, that's for sure.