Pocket-Lint: The Nexus is a strange category of device. Designed as a pure Android experience, it's a family of Google devices that's principally pitched at Android developers, but also popular with consumers.
Why is it popular? Because the Nexus devices are free from the clutter of manufacturer skins and customisations, so the focus remains on the core experience, rather than something slathered over the top. The Nexus devices are also the first in line to get Android updates, so benefit from Google's tweaks before the rest.