I've been trying Windows XP under virtual box for the past few days.
It works amazingly. Though as mentioned, having shared folders and usb support requires the version that includes closed-source code.
Except for some problems with the shared folders (sometimes they stop working), it's working great. And it's fast... a lot faster than I thought it would be (I allocated 512Mb for windows, though it also works very well with 256). Plus, the configuration is very easy.
By installing the Guest Additions ISO that comes with the package (open-source edition or not), you also get great integration with your desktop, because the mouse pointer is the same (plus better video support).
When on top of that, you take into account the fact that they opened most of their code, I can only praise this great piece of software.
I really only need windows for a single program, but virtual box saves me from having to dual-boot (plus always forgetting that I should have copied some files, then going back to linux, copying them, and back into windows... you feel yourself growing old while waiting for this process to end).
Wonder how the new VL SOHO performs in Virtual Box. Think I'll give it a try.