I use a 17" Macbook Pro. Generally the OS's font is okay. But when you are used to coding with fontsize 12, it becomes really small on my macbook. I use fontsize 13-14. It's big and looks really smooth.
Regarding screen size - the fonts are OK on my MBP too, but 27" is another story - you have to sit farther from such a huge screen. iCal and many other apps on my parents' iMac 24" are pretty much unusable because of tiny font sizes.
I am still on 10.4 though. One thing that bugs me is the size of the mouse cursor. It is too small for me -- I increase it and it all becomes jagged and ugly.