Up until the Nexus 6P, the Nexus line was really aimed at developers. As a result, oftentimes the hardware isn't the premium you'll see in other devices. Whether it be build quality (like the Nexus 5), camera quality (Moto and LG Nexus lines), or even more critical things like battery life, Nexus phones have been for those who want the latest and greatest software and a pure Google experience.
The 5X is a spiritual successor to the traditional Nexus (Google even says the "X" is for the core of Ne"x"us). Overall, a solid device with some big compromises. The 6P ("P" for "Premium") is a departure for Google. The Nexus 6P is a shift from a developer phone to a flagship killer. For example, the Nexus 6P has a more premium camera, solid metallic build quality, and solid battery life. at 7:00 PM every night, I have about 50% battery left. The feel is so premium and the price is fantastic (especially considering the base model has 32 GB of storage space).
Basically, if you are the type of person who looks at a Samsung or HTC device and thinks "Man, that software kind of ruins a great android phone", the Nexus is certainly for you. If you really dig the OEM skins or you want a stylus or you don't mind software that dabbles in different ideas (OEMs tend to introduce new features faster than Google, ie fingerprint and voice search), try the great collection of OEM devices running android.