Apart from VirtualBox being notably slower (and less complete in working D3D/OpenGL support) than f.e. Parallels Desktop, I can't say I've had any gripes or stability issues with it, barring a recent, short-lived bug that caused VBox to automatically power down an active VM as soon as it went idle. I've been using VBox on OS X for a bit over 2 years now, for development purposes related to Windows XP, OpenBSD, one or two Linux distributions, and even Haiku.