My ageing Galaxy S3 had a heart attack just before the Z5 came out (right in the middle of backing up my stuff - AAAARGH!) so obviously it died at just the right time.
I looked at a few phones (S6, LG G4, HTC One M9) and although I wanted the RAW photo support of the LG and HTC, I couldn't resist up to 2 day battery, 23MP camera, the fact it looks slick as hell AND dust/waterproofing. I really don't think Sony get enough credit for making a gorgeous-looking phone that you can take pictures of the inside of a fish's house with! It's truly impressive. Oh yeah, and 200GB SD support, too.
Anyway, add all that up and it was a no-brainer; none of the competitors had ALL those elements. It's fast and fluid too, could be slightly better perhaps, but more than good enough for my needs.
I didn't see the point in going for the premium for 4K - it's only 4K when displaying that content, although I was tempted. I couldn't hold on for the release as I needed a phone immediately. The screen is so good, it's hard to tell anyway.
Also loving the frosted back, although I'm paranoid that I'm going to crack it.
On the flipside though: it heats up quite a bit due to the 810 processor (still) but it's not uncomfortable, hasn't shut down at any point and only happens when I'm doing something particularly processor-intensive. So at least they have tempered the issues that made the Z3+ such a disaster.