I had an S3, then an S5, then the S7 edge, and now an S8+.
The GF got my S5 which is now a bit worn, so she got the older S7 which justified me getting the S8 hahaha
It's LOTS of small tweaks.
Personally I really like not having a home button, and it's on screen, meaning more screen to watch Por- I mean youtube.
The camera is a lot better, not just sheer quality but it seems to work faster and more effectively. Focus is good etc.
I can't offer much else, the S8 feels faster but could just be different animations etc
Battery life is the same despite being a bigger screen etc
I personally would only go for it if you intend to give the S7 away