The changes to the interface are pretty good overall. The Today page I feel like I'd use more now. Don't like iMessage though, tbh, and would like the old one back. New emoji design for the smileys doesn't seem better to me. Everything is wicked fast on the 6S, though. The update on the iPad Pro 9.7 is very good too.
Haven't seen it mentioned, but it seems to me that Night Shift has more granularity towards the Less Warm side of the scale now? I used to use it all the way to the left and it was still fairly warm, but that same setting is more blue now. This is great as I can more closely dial it to an accurate white balance and possibly run Night Shift all the time now (don't understand Apple's penchant for ~6900K default, maybe the new 7 display is better)
Also, time/date on the lock screen looks awful. Way too much weight on the font.
Also, also, Raise to Wake + Rest Finger setting = bliss. It's like a more refined Moto X experience that I missed.