Here is my impression from the 1 hour play test. First off, the phone is;
1. Fast
2. Fast
3. Fast
Now that I got that out of the way, here are some stagnant/negative stuff:
Ergonomics has gotten worse. I was unsure what to expect when I learned of the increased weight, and I did not know those ~0.2mm in every dimension combined with added weight would make this much of a negative impact. The 6s/Plus are among the more akward phones to hold in the company of other recent flagships. The 6s plus is an especially egregious case (192g!) and the thing feels like a mini-tablet, not a phone.
Display looks a bit brighter than the 6, but is still bluish. I thought this would have been fixed with the 6s but now I think that either 1) the panel Apple sources for the 6s/Plus has a technical limitation, or 2) Apple is cutting corners. All you need to do to confirm this is to compare the 6/6s' screens with the 5s/5c's screens, which are also slightly cooler (~7000K) but much closer to the standard 6500K.
The updated camera in the 6s is great, but not really different from the last year's camera that has fewer mega pixels. Compared with the S6's camera, it fell short in my indoor testing - not only in the outcome but also for the experience. For one thing, it is slower to focus and slower to snap than the S6. And the default camera app's functionality is too limited, IMO. That may have been fine and even desirable 2 years ago, but I am not sure whether that rationale still holds today.
3D touch in my brief sitting worked best in Safari. When works, it is nice to peek at hyperlinked content without committing to it. Elsewhere it is not intuitive nor reliable, and not compatible with 3rd party apps. This is the type of a "feature" that gets slammed as a gimmick/bloat when employed by other OEMs. Same goes for the "Live Photo," which is a copy of HTC's Zoe. Maybe Apple will prove me wrong with future updates, and I won't rule out the possibility of some amazing 3rd party developement turning the tide around, but it seems to me this is the kind of things that we usually do not give credit until it is proven otherwise.
Battery peformance did not look promising during the hour I played with it. But I ran benches and used cameras, both of which tend to drain battery quickly (especially slo-mo video), so I won't make a further comment on it.
Eager to read serious reviews from respected outlets.