VirtualBox running on a Windows 7 host with a Linux guest works great and improves each major release.
That being said, I've had severe issues with networking on older releases. Often the interface would work on the guest right after install, but would then completely fail the next time the VM was powered on.
I'm running into an issue where I get intermittent internet disconnection on the guest. I can't pinpoint what's causing but gmail will just start to time out. Wasn't there in Vbox 3.x though.
That being said, I've had severe issues with networking on older releases. Often the interface would work on the guest right after install, but would then completely fail the next time the VM was powered on.
It seems that has largely been fixed though.