My first impressions are as follows: (bought the 6s, coming from the 6 Plus)
1. 3D touch is a legit game changer. I've been thinking that we have hit a plateau of sorts when it comes to smartphone innovation, but this has already started to change the way I interact with the phone. And I love the keyboard/trackpad feature.
2. Touch ID is so fast I have to now use the power button to check the time, vs hitting the home button.
3. Phone flies, but still some hiccups depending on the app. I upgraded to iOS 9 on my 6 Plus, and was disappointed with the performance. The 6s absolutely screams.
Thats all for now! Hope everyone is loving their new devices.