ESXi has one major benefit that outweighs the others; IT'S FREE!Yes, it has plenty of other benefits, like running a wide variety of different virtual operating systems in virtual machines (it supports every platform but OS X, and that can technically still be "hacked" onto it).It's incredibly robust, has excellent resource management, and is easy to set up. It allows for importing of virtual appliances, or pre-made VM's, or a creation of your own VM's.
Room for Improvement:
The only issue I have with ESXi is that it is console (CLI) based. While I personally have no operational issue with this, as most of my training began with CLI based systems, some people might not want to configure a high availability virtual machine server via the Command Line.They're now marketing ESXi as VMware vSphere Hypervisor, which makes sense because that's essentially what it is. There used to be another product called ESX, which was a step between ESXi and vSphere, but they did away with that and rolled some of the features into ESXi.
All in all, it is an excellent solution that I have deployed in multiple locations and organizations.