I'm not sure how many tiles it would take for me to take DF, but I wouldn't take it with just a couple few desert hills. Usually though, if you start near desert, there's quite a bit of it so its usually an easy decision. DF is the only pantheon that I would call OP because it works on Flood Plains; and if you're going to settle in desert you're going to almost always have Flood Plains.
I have taken God King but only if there's no better one based on the land I have, and only if I don't think I'm going to get a religion because it is useless later in the game. But it is a nice boost early.
I used to take Fertility Rights but then I learned about the excess food part of it and its almost the opposite of GK, its pretty useless early game but kicks in late game. I haven't taken it in a long time because there's always something better that is based on the land around me.
@Tiberiu I know I tend to have smaller cities than most but 60 excess food in one city seems impossible. Or are you saying that FR is somehow additive and combines all your cities? Because that would change my opinion of it.
There's a good thread here on CivFanatics called Rate the Beliefs - Pantheons; you can see in that thread how wide opinions are on various pantheons. A lot of people seem to take universality into effect when thinking of a pantheon as good or not. I think of it as, if I were to take that pantheon, how much would it help my game as opposed to taking another pantheon. That's why I'd put DF at the top in a class of its own. The other pantheons I tend to take in relatively descending order:
Desert Folklore
Earth Mother - Faith for Salt, Copper, Iron
Sun God - Food for Banana, Citrus, Wheat
Religious Idols - Faith and Culture from Gold and Silver
Sacred Path - Culture from Jungle
Goddess of Festivals - Faith and Culture for Wine and Incense
One with Nature - faith from Natural Wonders
Goddess of the Hunt - Food from camps.
God of the Sea - Production for sea resources
Stone Circles - 2 faith from Quarries.
God of the Open Sky -Culture for pastures
Feed the World - Food for shrines, temples
God King
Having said that, its all situational. I took God of the Sea in a recent game because I had four sea resources in my capital that I knew I was going to work early. I also knew I wasn't getting a religion. Even though I lost it later because someone spread their religion to me, it was absolutely OP for the situation and helped me immensely in the early game.