I haven't done more than mess around with it. I got it running and then said, "great, what do I do with it?"
Same thing here, though I guess how open you are to a full Linux solution depends on the situation. I work in a student support position, and people are pretty happy to hear I can get them their files even though Windows won't boot. Linux has its uses. Another good couple are partitioning or memory testing.
Does VirtualBox have half-way decent 3D support on Linux? On my Mac VBox worked pretty bad on 3D.
I can only tell you my experiences with the Windows Version...
I just ran the "glxgears" command and got a pretty good framerate (according to the guest). I'm sure you've noticed the graphic lag that can occur with using VMs.
Those programs usually only focus on 2D, though for VirtualBox I'd recommend installing the additions.
It will allow the VM to adjust to any resolution (by window resizing) and it unlocks the "seamless" mode, which puts the guest OS on top of the host, allowing you to use your Linux programs and Windows at the same time.
Not sure how well that makes the 3D graphics work, though...