A big dump of my thoughts. Enjoy:
Hardware, Design, Materials
The flat edges are a welcome departure from the sloped edges we've had since the iPhone 6, but swiping in the edges is definitely much less friendly. Most people would use these for the back swipe, or the home bar swipe. This shouldn't really affect you if you were using a case, but the edge will catch your finger on this design. It also makes the phone look and feel much thicker; visually, it doesn't hide its thickness by reflecting light as well, and physically it doesn't nestle into your hand as nicely. There's certainly a lot more comfort and grip though, since it's able to dig into your hand. I've never experienced the 'bar of soap' feeling with the previous designs, but I know it's been an issue for a lot of people.
The buttons all give a solid click - they feel a bit firmer than previous iPhones, but that may just be because they're flat now. The mute switch on mine is solid, without any rattle or movement in either position. The SIM tray has a lot of friction, and is inset ever-so-slightly on my device.
Other fit & finish details: The display doesn't seem to be bordered by a visible plastic rim, so there should only be one seam for any potential dirt/dust to get trapped in. The frosted camera bump has a nice soft touch. The antenna lines aren't too attractive, but they're diagonally symmetrical at least. 2 holes for the mic; 4 holes for the bottom speaker; colour-matched screws if you get the white (it's they're silver across the board). The display has a subtly more 'plasticky' quality to it - like there's a hollowness behind it. The back glass, by comparison, has a denser quality. This could be down to the internal structure rather than the material itself.
This Year's White
**This is my favourite white they've done to date.** It's elegant but also cute. It's a warm white, that has a really subtle yellow/gold/pink glint at certain angles, but it's very very subtle. The silver edges are also quite warm - like a very light champagne colour. It's classy. It's completely different to the bright whites (XR, 11, SE2), or the dull white (8/8 Plus), or the silvery metallic whites (iPhone X, Xs).
Speakers
**The speakers are a significant improvement over the iPhone 8 and iPhone X.** Sound is 'fuller' (like more expanded if that makes sense), voices are clearer, and everything is better balanced. It's obviously still a set of phone speakers, but I think the audio quality is excellent.
If you listen to podcasts, voices sound a lot more natural. The iPhone 8/X separation has always caused a sort of garbled/digital effect on voices, but it's nearly perfect on this 12 Mini.
Display
The display is a fantastic size. While I wouldn't say it's comfortable to reach the edges, it's much easier to do compared to the 5.8" iPhones. The lighter weight makes is easy to shimmy the iphone across your hand if you have smaller hands.
True Tone seems a bit more aggressive than it was on my iPhone 8. Compared to an OLED iPhone X, my 12 Mini has a more yellow/green cast with True Tone off, and is significantly yellower with True Tone on.
Downsampling/Display Scaling
I've quickly tested the **downsampling** with a Netflix test pattern (1080p, Season 2 Episode 8). I'm definitely getting some artifacts on the centre horizontal pattern - it renders as blocks rather than discrete lines. I'm not sure that this is caused by downsampling though, as I thought that would usually cause a Moire pattern. This is tested with True Tone off, and any overlays disabled (no Assistive Touch). Tapping the display to show the Netflix UI has no effect on the displayed image - I believe the 6s Plus rendered video differently depending on if the UI was showing.
The
rtings sharpness pattern definitely causes a Moire pattern in the pixel grid on the bottom left, but I'm not 100% sure how the Photos app handles its scaling.
I don't see any issues with text sharpness, or horizontal lines in apps, so the downsampling shouldn't be an issue in regular use.