The only thing I must say to defend my preference for a native speaker of French as a teacher is that I don't really enjoy learning a language per se and a "proper" French teacher can open doors to a whole new and exciting French world.
I love the cultural side; what the language does for your understanding of the social, artistic, traditional aspects of the life that the language describes.
To give a very basic example, I soon learned, practically right after arrival, to ask for "un petit kilo" when I don't want quite as much as a full kilo and it's the sort of taken-for-granted statement that a native speaker would use but which a foreigner person would not necessarily understand.
Another example is how you call where you live a "maison" (when you aren't talking about "chez") regardless of whether it's a house, a flat, a caravan, a shed, whatever. I remember thinking that very strange but it's little bits and bobs of information like that which I find fascinating.
A language to me is a lot more than conjugating verbs (which lots of people I know seem to think it is) and being able to put pronouns in the right order Yes, yes, I know all that's hard enough but the reward for all that effort has got to be something rather more than the sum of the parts assembled together?
Apprendre une langue, c'est faire un voyage différent chaque jour.
from Fle pour les curieux