What I think
If I were to buy a new phone now and didn't get an iPhone 6, I'd buy this, easily. In 64GB or 128GB -- never a 16GB (those smaller sizes barely hold enough photos and videos to get by if you're the type of person who doesn't delete). If I had an iPhone 6, I'd wait until next year...or, find a way to sell my phone and trade up without it costing me quite so much.
I think Apple, and the whole phone industry, are trying to change the "Should I upgrade?" question bit by bit. They're trying to make it an "every year" thing, not an "every two years" or "Should I buy something new?" thing. Phone plans like AT&T Next, or Apple's own yearly plan, are almost like leases. You can trade in your old ride and seamlessly move to a new one.
The 6S fits that model well; it's enhanced and improved. Is it shockingly new or a vault forward? No...except for the concept of 3D Touch, which is brilliant hardware still in need of software which will make it truly shine. That software, from iOS itself and apps, will come, but maybe not right away. 3D Touch could build a new style of touchscreen app design, bleeding into iPads, the Apple Watch and maybe influencing the rest of the industry. Right now, just a week in, it's still baby steps.
This iPhone is built for the future, but in the present it's mostly a nicely improved iPhone 6.