I can say that ever single company I've worked for has used VMware for their virtualization solution.
I also use it for my business. It's pretty much ubiquitous, if you ask me.
That said, I don't expect it to stay that way forever.
<heavily opinionated part>
Some folks say KVM performs better, but I haven't seen much difference in my experience.
That, and KVM has little/no polish. Certain things are much more difficult to do with KVM, like create a VM with anything other than a NAT interface.
There are LOTS of tools available for VMware.
There just seem to be a lot of things that make VMware one of the easiest solutions to go with. I think ubiquity is part of that. If you're running something on VMware, and you want to relocate VMs, it's easy to do so.
OpenStack - also messy, and not very easy to learn. I'd love to learn it, as I hear folks with OpenStack skills can make bank. It seems like a jumbled mess, though, and I have no idea where to start. </heavily opinionated part> :D
Initially, my experience with virtualization was just desktop products. Now I run a vSphere host in my home office, and admin several clusters for work.
I have read elsewhere that KVM is basically just the plumbing for the virtualization, and I was trying to use it with a front end like virt-manager, and KVM is basically just the back end piece.
I'm prepared to be corrected/educated on this. Open mind here.
Definitely have a bias to VMware, but I'd like to know more about the competition.
Can only speak from experience within US manufacturing. Don't apply this beyond that.
Was formerly part of a US company that was pushing a new product hard towards a cloud-based model. They were expecting a particular foreign customer to go for it.
The customer said "NO WAY. EVER."
Small and medium IT shops probably lean towards an open source based solution like KVM.