Coming up on 24 hours, these are my primary thoughts:
1) The size/weight is so great. I've already "lost" the phone in my pocket and got up to fetch, not realizing it was already on me. So nice to use in bed.
2) Feels like it should never have a case on it. Your hand just engulfs this thing, so it's hard to imagine sticking a case on it. The 12 Pro, on the other hand, always seemed to be needing a case. I am seriously considering returning the mini case.
3) The MagSafe wallet seems to work well on the Mini. Wasn't impressed with it on the 12 Pro. Mainly because it goes right to every edge, I think. Was going to return it, but may keep it now.
4) Text size hasn't been an issue (I'm 48). Battery life has been fine, but really hard to judge on any phone in the first 48 hours.
5) Camera is pretty good, but not great. Depends on the shots. I've done a lot of shots where I take the same pic back to back with the mini and the 12 Pro. On "normal" shots with no zoom, the mini looks great (which it should, since it uses the same lens). But any shots where you zoom look much better (tighter, more crisp, better focus) on the 12 Pro.
6) Love the aluminum sides. I have the blue phone. The back is ok - total fingerprint magnet.
7) Screen brightness has been a non-issue for me.
8) Since I've been using the phone naked, I haven't experienced the Lock Screen issue once.
At this point, I'm probably keeping the mini and returning the 12 Pro. The size is something you feel literally every use - the camera differences are rare. If there was a 12 Pro Mini, I would just give Apple a blank check.