I have used them over the years. A bit tricky to setup but worth it once you get over the hump. There are also security issues, etc but there are step-by-step details on what to do in the help. You may have to disable/enable various permissions on the VMs, open ports through Windows firewall, etc. But like I said, definitely worth it.
I would test against 17 languages and 29 currencies for 5 variants of a product over four VMs concurrently and, in this case, since the products were different skinned versions of basically the same thing, it shaved it down to a quarter of the time.
There's also TestLeft which can do similar distributed testing but is more programmer-centric of course.