Very fast, very responsive, no worries about fsck on boot with immediate recovery. Works great for servers, VMs, NAS, laptops, and desktops. Uses the same packaging system as FreeBSD so you have over 20,000 packages available for easy install. Best of all, the community. Small, tightly knit, really intelligent developers. If you ever have a problem, you can bring it up to them, and the odds are that if they're not overly busy or it's critical that it'll get a fix in within a day or two.
Very probably the fastest Unix available. Developers really take "KISS" seriously and the pace of development is nicely paced. Once HAMMER2 is available and ready for prime time, this will be the OS to take cloud computing to the next level.