I will reserve judgement on Unity until it works properly, but the current Natty version seems to me to have a lot of promise. Re VirtualBox, Unity works with the current svn Guest Additions, and I think that the Ubuntu maintainer has backported the change to the packaged 4.0 Guest Additions in Natty. Unity is not very stable in VBox though, and I don't know which of the two is to blame for that (or how the blame is to be divided).