A powerful research environment. Especially if you're doing research: you can focus in the work and find a variety of ways to do it. Alongside with pkgsrc almost everything works out-of-the-box. It also has got an impressive and friendly community. Other aspects have been written in the other reviews...
Con: the only "con" NBSD "has" is this: if you're used to others handle your problems, used to solve things by clicking and you're lazy enough to read, NBSD might be not for you.
NetBSD is a delighting experience to work with, even if you're just browsing the net.
This is what can be accomplished in the open source landscape. It's truly amazing that this community can make a modern operating system that works on outdated architectures. Well, what can I say next?
If you have some old computer that needs a good ol' kicking, get NetBSD!
It's BSD! A fast, easy to use and polished operating system. Nothing more to say, it's amazing!
NetBSD is my daily driver. Excellent piece of software. I was a GNU/Linux user since forever and I switched to NetBSD few years ago without looking back.
There is a helpful community, solid development method, strong technical community, conservative model, focus on correctness (thousands of ATF unit tests) and usefulness. There are commercial vendors around this system and the portable package collection (pkgsrc).
This system is improving rapidly and catching up after competitors, not necessarily being the first it usually produces better solutions.
NetBSD isn't an OS that will hold your hand during installation and set-up, but it's great for server applications and pretty good for desktop use. It also has a great source-based package manager, pkgsrc.
Tried NetBSD 7.1, but couldn't get Networking to work even though I tried to follow the directions. I do give credit for NetBSD's success with embedded devices and that it can run on a huge number of platforms. The main flaw is its very difficult to get it to work properly on the desktop, much more so than Slackware. With the exception of GNU/Hurd and Minix, NetBSD is the oldest FOSS operating system still maintained. A GhostBSD style variant of NetBSD would be welcome!