I did buy flashcards once when she was two and starting to read because I was excited by it, and the first time I showed them to her, she knew every word and so that was it for us.
That's how my teaching attempts go to. I find he already knew it or he learns it once and that's that. 
I have to say that these debates are making me want to get out his addition flashcards. I bought some flashcards on sale thinking that we would use them eventually. I tried them out for fun and HE LOVED THEM. I felt so ridiculous using flashcards that I put them away again.
I'm smiling over the talk about flashcards.  I hated them as a child and really hated when my mom "hothoused" me in forth grade when she was told by the teacher that I was doing my multiplication work at school by super-fast finger counting. 
So, along comes DS. He's a mathy type who has loved numbers as long as I can recall.  He would/still demand(s) counting at night instead of a story to relax.
I thought, "He will like flashcards!"  Um, he does.  Just not in the way tradition dictates they be used.  His idea of flashcard use is to throw them around the room.  Pick a few up and look at them and throw them down again.  Maybe place all the 1 times cards in a row.  Maybe place all the cards with an answer with "4's" in them in a row.
Is he learning?  Yes.  Just how he wants.