Virtualbox?

I want to run some software that REALLY doesn't like running on my Windows 7 PC, and I really don't want to upgrade to Professional just so I can run one program under XP. But I think I have an old XP install CD somewhere, and Virtualbox looks like it'll let me run XP under Windows 7 (or possibly on my Linux box) - does anyone have any experience of it, and know if there's any down-side to it?