Ranked in these QuestionsQuestion Ranking
Pro It's real
Under NetBSD csh is csh not tcsh; also vi is real vi not elvis, nvi or vim. It's ideal for purists.
Pro Adhere to the standard
It adheres to traditional Unix and new defined standards.
Pro Architecture portability
It's the most portable OS in the world when considering what architectures it can run on. It runs on very wide range of hardware, from toaster to satellites. This of course does not mean it supports drivers for many consumer facing products making it a difficult solution to just boot up and use when compared to other OSs.
Pro It's Open Source
It's open source with a BSD License, which is much more business friendly than GPL. It's the real ancestor of Mac, that is being used nowadays.
Pro Clean source code
It prioritizes source code cleanliness over anything.
Con Lack of drivers
It lacks drivers for some new devices.