Xen is VMWare's #1 competitor so it can be just as ubiquitous. So it's worth it to learn it.
In the enthusiast space, maybe
... not anywhere else.
In the enterprise space (where the money is made), it's Microsoft HyperV. Nothing else has the stability and infrastructure to compete with VMware's enterprise lineup.
You shouldn't concern yourself too much about Type 1 or Type 2 hypervisors either. At this point their performance differences just aren't that great considering the amount of hardware that is under most VM servers, and the need to pass through hardware into the VM is only required in some special cases. In addition the software for a lot of the Type 2's (Virtualbox) has become so good that it has blurred the lines between them anyway and can perform some of the same tricks of Type 1 hypervisors. So don't worry about it.
Once again, I suppose in the enthusiast space. Sure, I wouldn't recommend every guy with a PC to run ESX. But if we're talking corporate boxes, ESX can handle more load than anything else, anywhere, period, because of its Type 1 maturity and efficiency. I think your phraseology omits the framework of your point of view.
And even in the Type 1 space, VMware has a significant performance lead over Microsoft, showing I think two things: 1) their much greater experience in the space, and 2) their ability to go truly light-weight, unlike Microsoft who even in a Type 1 hypervisor is compelled to keep it tightly coupled with Windows.