Tossing a VM between machines

[TheFaQQer]

I was thinking that if you had your dev-PC running as a VM, you could simply throw this single VM file between your desktop and laptop when you have to travel rather than muck about with SVN, Dropbox, yada yada.

Is this feasible - is a VM image a single entirely self-contained file?
Are there even any tools to sync a VM image between multiple machines so you don't have to copy the whole multi-Gb file each time?

I do it with VMWare - copy the files onto a USB3 HDD for using on my laptop, then when I'm back I copy them back onto the desktop machine.

Works between Windows and Linux as well with no issues - been doing it for years, as I have a Windows XP virtual machine for development work but prefer to use Linux for my desktop machine.