SmartOS is a hypervisor derived from Illumos, the open source evolution of OpenSolaris. I have been using Solaris since early 90's and I still consider this Operating System far more "serious" and enterprise ready than Linux.
Most distributions based on Illumos are complete Operating Systems. SmartOS is a different beast. A typical SmartOS deployment consists in a thin hypervisor running multiple Solaris containers. Something similar to Docker but more capable and far more secure.
As a hypervisor, SmartOS boots from a read only media like a USB flash drive, a DVD or PXE. The image is quite small, less than 300 Megabytes. There is no personalization stored in the boot media, the server boots from that media and tries to read the configuration from the local harddisk. That is a quite nice approach: upgrading the hypervisor is trivial and failsafe because you can always boot from the previous version, you can not bork your server with a problematic upgrade. Being 300 Megabytes in size, you can keep quite a few versions of SmartOS around, just in case.
I like the idea a lot, but it usually requires direct access to the physical infraestructure. You need to replace the DVD or you need control over the network in order to run PXE securely. Some hosting providers have the option to plug an USB flash drive as a one-time option, for a price. The problem here is that you have to pay and you will need to pay again if you migrate to a new server in the future. It could require a KVM too (more money) in order to configure the BIOS to boot from a USB flash drive. Also, many cloud providers doesn't have this option. You are restricting the choice of providers if you go thru this path.