After install, pretty much everything works out of the box, except one annoyance. The screen resolution was stuck at 1024 x 768, which on a MacBookPro is annoying when you've got 1280 x 800 to play with.
Selecting System > Preferences > Monitor gives only 1024 x 768, 800 x 600, 640 x 480.
You need to install two things. First open a terminal window (Applications > Accessories > Terminal) and run the command
sudo apt-get install dkms
Put in your password, and it installs.
After its finished, install the Additions:
Right at the top, on the VirtualBox menu bar, select Devices / Install Guest Additions. You get the AutoRun prompt, so accept, then after click Run, and put in your password when asked.
Follow this through, it may take a few minutes and then restart. The 'power button' upper right.
After restart, initially things will look worse! You'll get 800 x 600. Don't worry. Go into full screen mode by pressing Apple-F.
The window will resize automatically to fill your screen ie 1280 x 800. You can then back into monitor settings and the new resolution will be there. When you press Apple-F again you'll run windowed mode, which is actually my favourite.