Overall, after spending all this time with the iPhone 6s I can’t find anything really wrong with this phone.
On the contrary, the A9 SoC is a huge jump in performance even relative to other SoCs on the same process node to give impressive application performance.
The storage solution is unlike anything else in mobile that I’ve seen so far.
The camera’s overall user experience is just about the best that you can get on the market. 3D Touch is a big improvement in user experience, while TouchID v2 and always-on Siri are worthwhile improvements in user experience.
The only real issues I can think of are that the iPhone 6s doesn’t have OIS and that the base SKU is still 16GB of storage. To be fair, the 16GB SKU can become a noticeable user experience issue if you're constantly dealing with the limits of this storage, and the jump from 16 to 64 GB feels like it's simply designed to encourage buying a more expensive SKU. There are arguments that users that don't really take a ton of photos or videos and stream all their media will be fine, but it's still a user experience problem in this day and age.
However, despite these issues I would argue that the iPhone 6s’ are the best phones you can buy today.