How long has it been since you looked at Ubuntu's fonts? I use both OS X and Ubuntu, and Ubuntu is at least equal to OS X these days IMO.
Ubuntu and OS X are not even remotely comparable from a typography standpoint. Using Freetype (the Linux/X11 lib for font rendering), tracking and stem thickness is inconsistent, ascenders and descenders get cut off, etc.
The Freetype team puts forth a valiant effort, but until they're allowed to use Adobe/Apple/etc.'s patented hinting algorithms, there's no comparison.
And yes, I wish Ubuntu would at least make DejaVu the defaults for everything out of the box.
sudo aptitude install ttf-mscorefonts-installer
Still.. even if you do install the right fonts, the results will be the same.