I'm currently a regular user of Virtualbox (Ubuntu 10.10/Xfce/Compiz guest on Win 7 64-bit host) and while it's mostly sufficient, the 3D acceleration still has a tendency to crash my VM. With 4.0.2, the crashes were only caused by full-screening an app in the guest. I updated to 4.0.4 the other day, rebuilt my guest addons and now I'm getting crashes for no discernible reason whatsoever. Yay.