I return my iPhone 15 Pro Max and stay with my 11 Pro Max. The 11 Pro Max has better battery / efficiency, better camera in certain conditions, feels way better in my hands, better sound quality. I was happy to change after 4 years, but I am amazed (in a bad way) how little has changed. Apple Support was completely useless. There was obvious hardware damage on the main camera and they don't even care. In my opinion: Price/Value is bad since iPhone 14 - at least for the Pro-Models. Sad to see drop in support and product quality. Tested 13 Pro Max from a friend vs. 15 Pro Max and yeah the 15 Pro Max has some better photos, but some pictures are way worse. USB-C doesn't work with the original Apple Thunderbolt-Cable I use to run my Macbook Air on big display. iPhone just doesn't show anything. Another cable worked fine - but total useless to use external display if they don't have a desktop-modus where I can use my keyboard and mouse without problems or changing 100 settings to adapt.
All in all a nice (useless) upgrade. The phone looks good, the functions aren't worth the upgrade (for me).
iPhone 11 Pro Max just feels like its something "finished". iPhone 15 Pro Max feels like it's not fully thought through. Like the "closest focusing distance" of the main camera. There is a gap between main camera lens and the macro-modus. If you shoot photos within that range there will be blurry and unsharp photos (on the edges) as a result. You need to keep more distance to make sharp photos of newspapers, documents, etc. and crop. Same for photographing other close-distance objects.
Worst upgrade in my life. Used apple devices since 2012 and I'm now looking at options outside the apple realm.
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If you really want to upgrade from an iPhone 11 Pro Max (or lower), I would suggest to have a look at the iPhone 13 Pro. The biggest gap in differences is closed with that upgrade. There is little change from 13 Pro to 14 Pro to 15 Pro and the 13 Pro has good battery, really good photo quality, good display, good price/value, good speakers, good (enough) CPU.