Based on what I know, the popularity of Linux on the mac platform is rather small. We're talking a minority within a minority here.
However with the increased adoption of Mac OS X, it is opinion that in the long term, people will be interested more in Linux and BSD on mac hardware.
Older machines in particular, incapable of running darwin which people are still seeking a use for..
A problem with Linux on newer machines is that they often cannot fully utilize all the hardware mechanisms of the system or depending on the distro, they require a lot of work just to get running on a newer system.
I have a great deal of respect for the open source community however, they do after all only recieve a small amount of support from Apple (based on my knowledge). They're also delivering software that empowers a driving movement for alternative operating systems and solutions on some really kick ass hardware.
Anyway, back to my question. What is the best distribution overall, or if that is a question too simple to be answered, what are the qualities one should look for in choosing one?