I find the explanations of phi very good, and I have little suggestions to add:
Metapost:
I was using Metapost sporadically bevore I stumbled upon Asymptote, it happened because I did not like Xfig anymore and really thought I could try something else, I first found that exporting from Xfig into .mp (Metapost source files) is a nice thing.
A friend of mine showed me some examples of using Gnuplot also to export the code for the images to Metapost, he was quite satisfied with the result.
Asymptote:
Some nice examples if you want to get familiar with Asymptote you can find looking at the the gallery.
Of course the documentation gives worthfull suggestions, too.
So, if you like the Gallery, at
http://asymptote.sourceforge.net/gallery/
then this gallery ist really a blast!
http://piprim.tuxfamily.org/asymptote/
Document Classes ⇒ Metapost Asymptote anyone? VS pstricks
