I'm a reseller of this solution.
I use vCenter if I need to do administration on a centralized layer when using multiple VMware ESX servers. I'm able to manage the VMware environment where all of the VMware machines and multiple ESX are hosted.
There are multiple versions of this solution available. We have more than 100,000 people in my organization. We have over 500 customers. If I look at the landscape, I have more than 300 customers, whose landscape in terms of number of servers is more than 10,000. For the virtualization, at least 60 or 70% is running on VMware.