I don't have tons to add, but I was recently in the market for a used pin to start my collection.
Demo Man was often brought up to me because of its flow. Additional points made to me by several folks included:
* Easy to learn how to fix this machine. Great machine to start with for a beginner like me.
* Lots of available parts available.
* Relatively easy to find great quality ones at a fair price.
Never having got it I am unsure how accurate these are, but I certainly heard it several times from various sources.