I'll use this review to mostly say what I'm not a fan of in this particular iPhone model.
But first, what's good.
Battery life is excellent so far. Charged to 97% 16 hours ago and I'm at 77% currently - though I have not used the phone much in that period. Sound quality is a touch better than on the 6S, I think. Portrait mode on the camera is great. Everything else is more or less the same. The chip is faster, but my old phone wasn't slow in the least, so there's no palpable difference really. The screen resolution is the same, as far as I'm aware. I haven't had occasion to test the water resistance, but it's definitely good to have.
What's not so good.
The weight. This is supposed to be 194 grams, which is more than 30% greater than the weight of the iPhone 6S, my old phone. It certainly feels like it. Add about 25-odd grams for even the lightest case. (And you do need a case - this is a super-slippery phone.) On the plus side, you're pretty much always conscious of it being in your pocket, and you'll probably notice if it fell out.
The size feels somewhat monstrous, at least for now. 6.1 inches is massive, coming from the 4.7 of the 6S. Combine the extra size with the extra weight and I feel less secure and comfortable holding this phone in one hand than I did my old phone. It generally feels less "manoeuvrable" than a smaller phone. And my hands are reasonably large. That translates to a somewhat poorer experience overall. Anyway, I can't complain - I knew about the size before making the purchase, but just a note for people who are thinking of upsizing. The good thing about that is that I'll probably use the phone less - which is something we all need to do, in general.
Face ID has been a bit of a pain for me so far. Doesn't work when you have sunglasses on, or in low-light conditions, though it is supposed to "learn" your face and get better as you go along. Three weeks in, I can't say there has been much of a change. Would have loved to have an option to immediately evoke the passcode input, which now takes more than two seconds to come up after Face ID fails. The only thing I like about Face ID at the moment is that it lowers the volume of alerts if you're looking at the phone.
When I'm out walking with this phone in my hand, I usually take care to have the screen angled towards the ground, so that the sensor does not "see" my face somehow and bring the screen to life. But it does do this off and on. Worse, when the screen comes alive like this, if a song is playing, my thumb or fingers end up skipping or re-playing the track.
I don't know if the Tru Tone on my unit is wonky but the warmth of the backlighting seems to change more often than it did on my old phone.
The absence of a headphone jack is, as millions of people have observed, a severe pain in the neck for those who don't want to use wireless earphones that make you look like an alien and are based on Bluetooth, which feels past its time as a protocol.
Spotify is quite buggy, but then it was nearly as bad on my old phone, so that's probably a Spotify problem.
People talk about the massive bezel, but to me, unless someone is pointing it out to me, I don't really notice it. It'll be different for those who have used the X, I suppose.
I can't say I'm regretting having bought this. I probably wouldn't have bought it just yet, if I hadn't planned on giving my 6S to a family member. I hope it grows on me.
UPDATE, 25 June 2019: Three months in, I have got completely used to the size and weight of this phone in the hand. One-handed operation is still not safe or convenient but I have stopped thinking of it as a massive phone. When I hold an iPhone 5s in my hand, it now feels impossibly small.
Face ID still does not recognise my face when I'm wearing dark glasses. It does usually recognise it with or without regular glasses on, however. But sometimes, in low-light conditions, I need to move the phone up to face level before it will unlock. Apple might need to consider allowing users to add more than the one alternative look in Face ID that it does at present before this problem is solved.
The problems I had with the screen waking up have almost vanished. I suppose it was the edges of my fingers touching the screen inadvertently when I held it in my hand that woke it up.