Apple made a nice phone here. The two key features that may entice you to upgrade are the Taptic Engine experience and the Dual Lens camera... otherwise, if you're happy with your iPhone 6 or 6s Plus then keep them.
Taptic Engine provides a new level of user experience than ever before on an iPhone; Portrait Mode and the dual camera are probably the things that should tempt you to upgrade
Design is nice but dated; iOS 10 picks up a ton of new features but the UI needs more user customization