What kind of flavour do Devil Fruits have?
Devil fruits taste awful.

This is evidenced by the few occurences we've seen where a fruit has been eaten.

When Law eats his fruit (episode 706):
 

And when Kaku eats Ushi Ushi no Mi Model: Giraffe and Kalifa eats the Awa Awa no Mi during the Water 7 Arc (Episode 271): 



During the same scene Jabra was trying to convince them not to eat it by saying that it tasted like crap:



Based off the several examples of people eating it and saying it taste like crap I can definitely answer your question of how they taste with: Unlike their normal-fruit counterparts, they taste awful.