Say what you will about Apple, they really do know how to design a lovely looking phone. The iPhone 6S is the latest iteration in their never ending quest to change everything, again.
The 6S follows through Apple's previous strategy of reusing the design of its predecessor. The 'S' moniker is there to indicate a performance update (Speed, see what they did there?), and boy does it perform. The Apple A9 CPU is a beast. Its single core performance is blazing fast, and its multi-core's none too shabby either. Without getting technical, the 6S is the fastest phone you can get on the market today.
iOS9 comes preloaded with the 6S and is optimised for the larger screen. I had been testing it on a 4S previously and found it highly unusable on a small display, but on the 6S - it works just fine. It just isn't for me. As someone who loves the design of Windows (Metro) or the fluidity that Android provides, iOS's garden wall restriction hampers my enjoyment somewhat. Still to some, it is perfect, understandably, and I won't slate anyone who prefers it.