Just received my 12 Mini. I am very impressed, coming from an iPhone 8. I also bought a Spigen clear case and it shows off the blue beautifully.
The phone is very fast and responsive and the features, form factor and quality are well worth the price to me. For my use case, I find the battery life quite acceptable. During my 14 hour day of use, I still had 55% battery. After a little more that an hour was charging back to 100%. Out of the 5 other iPhones I've had ( OG, 3G, 4, 4S,5S), this ranks as one of the best I've had.
Will this form factor go away? Quite possible. The SE is a price phone with similar attributes that apparently have cut into the mini sales. Depending on need and features, I am usually attracted to a smaller, less expensive purchase, as I am not a power user or are in need of more expensive camera features as I prefer using my DSLR for those purposes. With that said, I found the 12 mini to compromise between the price phone and the more elaborate ones. Each to their own need. I don't always upgrade to the latest and greatest for budget reasons, but I was able to get the great trade in deal from ATT.