We have been using Vagrant and VirtualBox heavily and the new Docker for Windows/Mac is making us reconsider that since you can't easily use more than one hypervisor on the same dev machine without some hassle. We might be building our Vagrant boxes for these other hypervisors soon. VirtualBox still seems easier to work with but there isn't anything much exciting happening with it lately.
Let's see...
(Yes, I know there theoretically exist different Vagrant backends, but Vagrantfiles and public images are married to specific backend so all the reasons to use Vagrant tie it to VB)
I think when Windows Nano Server is available I'll give it a try, my Win7 image is almost 70GB...