To begin, I love the Apple eco-structure and have enough Apple devices at home to open my own Apple store. Being able to share purchased apps, songs, movies, etc. with my family members is great, and utilizing iCloud to synchronize things between my phone (iPhone), tablet (iPad), and computers (Macs) is the icing on the cake that keeps me locked into Apple products. With that being said, the 6s is a great phone, but may not be for everyone, depending on what phone you already have.
The new features, such as 3D touch, a faster Touch ID, 4K video, and the like are nice, but are not that revolutionary (or evolutionary) compared to the iPhone 6 I had previously. 4K video takes up a lot more space and, from what I can tell, still only uses H264 (as opposed to H265) encoding, despite the fact H265 is apparently used for FaceTime. I have had the phone for a couple weeks and have yet to utilize the 3D Touch feature. (Then again, I have something similar on my MacBook and have yet to use its Force Touch feature on the trackpad.) Unlike some of the gestures used with a multi-touch display, the 3D Touch is not that intuitive for me. I rarely take selfies, so the screen flash is not a big deal and, while some people are saying the phone and camera is faster that previous editions, I really have not noticed much of a difference. The Touch ID is indeed faster at recognizing my finger, which has actually been a problem as pushing the home button typically simultaneously unlocks my phone, which is not always what I want (especially when I am just trying to see the time or want to use the lock screen feature to open my camera, which is faster than navigating for the icon on the home screen). iOS 9 has some nice features, but you can run it on older phones with the same features. Oh, it also comes in rose gold now, which doesn't matter one little bit to me as I keep my phone in a case all the time anyway. Lastly, I have not even played around with the Live Photos feature. It looked kind of cool during the Apple keynote (made me think of Harry Potter, actually, with the moving photographs in the Daily Prophet), but the "pictures" take up almost twice the space, you have to remember to hold your phone still for 3/4 of a second after you take a picture, and the video it records both before and after the picture is at 15 fps, which apparently does not handle motion very well. If I want a video, I'll simply record a video. (I can always extract a single frame from a video, if I really wanted a still picture.) Again, it sounds like a neat feature, but I really have not had the impetus to turn it on.
If I had an iPhone 5s or earlier, this would have been a great upgrade. Just the extra screen real estate to 4.7" would make it worth the upgrade. If you already have an iPhone 6, however, I would think twice about this particular upgrade, unless one of the new features is more important to you than it has proven to be for me.