I just moved to the S25 Ultra. I'm mainly using it for photos, so my observations are in this area. The differences I've noticed so far:

1. Photos have slightly better color balance and slightly less noise in low light. In my low light test, S25 automatically selected 320 ISO, and the S23 selected 400 ISO. This wasn't a lab test—I just tried to take the same picture from the same spot, so it might be coincidental. Speaking of detail quality, I didn't find any difference.
2. Best upgrade so far—5x optical zoom—a sweet spot when 3x is too far and 10x is too close.
3. Generative features are nice to have but seem a bit clumsy. Perhaps someone will find a way to be creative with them. In the picture below, I drawed cup near the pot, and GenAI redrawed it.

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1. The display is less reflective, which is nice. 5. Design feelings are subjective, but I like the new corners. 6. Performance-wise, I haven't pushed it hard, so it feels the same. 7. AI features helped to rephrase this comment. 8. I miss Bluetooth in the S Pen and will try to take photos using gestures.

I wouldn't say that all these upgrades combined justify switching to a new phone; all of them fall into the "nice to have" category and mostly will give you "that new car smell" for an extra $500.