I don't believe that they are a myth, but I do think that there are different interpretations of a "craving".
With Aliya my strongest one was for Twister ice lollies. I ate at least 1-2 every day throughout the 3rd tri. After my first box, I couldn't remember where I had bought them from, and actually had a stamping my feet tantrum and burst into tears in Tesco as it was the last place I could think of to look and they didn't have them. Turns out it was the local Spar that sold them, which I remembered a few days later and rushed out immediately to buy several boxes! After Aliya was born I couldn't face eating them at all, I ended up with a box sat in the bottom of my freezer for months before I threw them away. I've not eaten one since!
This time i've had very strong urges for things, satsumas (clementines etc) in the first tri - one of very few things I could eat without feeling sick, so I ate bags of them.
In my second tri it was custard. there was a period of about 2 weeks where I ate a pint a day. I would just have to think about custard and I could smell it, and taste it already, and then I had to go make some. And it had to be the stuff you make with milk - my mum once made me some from a packet with water and it was just no good - I had to go and make the proper stuff!
I also seem to have a strong desire for MacDonalds Cheeseburgers, which I normally don't like. I don't like the mustard they put in them, but that is exactly what appeals at the mo....weird!