Got the 64GB Space Grey today.
It will replace a 4.7" iPhone 6 and 3.5" iPhone 4S (still useful in some situations). I tried loving the 6 for 1.5 years, but simply can't. It has many nice aspects, but for me those are outweighed by too many annoyances when using it. The 4S is still a great device, but it's getting too slow and pretty much every app can't be properly used anymore on a 3.5" screen.
Impressions of the SE so far:
For me the 4" form factor and design of the iPhone 5/5s is actually completely new and right in the middle, which turns out to be really nice. I remember that I felt like I could do so much more with the 6 due to the extra screen real-estate. And while that's still true in some scenarios, being able to properly hold a phone and reach everything turns out to be so much more important. I feel like I'm way faster doing pretty much anything, and without any effort. Not balancing the phone in your hand to touch something is pretty amazing. I'm still afraid I'll drop it (newness and all), but given that I drop my 6 about once a week and almost never dropped the 4s in all those years, I think it'll be fine.
I noticed a yellowish tint at the top during setup on the white screen, but only from a certain angle and because I read about it here. Haven't noticed it since. Overall the screen is more yellowish compared to the 6 (which is very blue). I guess it's good for the eyes, and without another phone/screen right next to it I simply can't tell.
The power button rattles when I shake the phone. Haven't heard it once during normal use though. Even went jogging with it while holding it in my hand, but that's not enough "shaking" to rattle it. So for me a non-issue.
Don't know what kind of CPU it has, and won't check. Whatever it is, it's fast.
Bluetooth earphones work great.
The screen attracts dust like crazy. Not sure if it's the newness, and the static-ness or whatever will lessen after some use. Not that big of a deal though; a quick swipe and it's gone. And in comparison the screen of the 6 is a huge smudge-magnet. Not noticing many finger smudges so far on the SE.
The screen quality itself is definitely different than that of the 6. I'm not sure that I'd call it worse per se. Comparing them side-by-side the 6 has my preference. Probably due to the rounded edges, the screen being closer to the glass (at least it seems that way) and the larger screen. But I'm pretty sure every larger screen automatically feels "better". In practice (i.e. not using two phones side-by-side for constant comparison) I've completely forgotten the 6 and the SE screen looks awesome. It also looks way better than the 4s, that's for sure.
The vibration motor is loud. I believe the 6s uses the element that also makes the 3D Touch work, and the 6 still has a normal motor. People seem to prefer the 6s's vibrations, but I've never felt those. The 6's motor is way less pronounced though. Not sure I want to lay the SE on a table, because buzzes will be heard across the room.
Not much else to say for now. Unless something goes terribly wrong in the next couple of days or there's some big hardware defect I haven't noticed yet, I'm pretty sure this will be my favorite Apple device to date. The design, size, using it, ... I love everything about it.