my DD has never ever had long hair, so I don't know about cutting long hair off, but she has a concave bob. Really short at the back and longer at the front. This came about as she hates to have her hair brushed and as it was so fine it was always so knotty, so I figure I have bigger battles to fight and it has been in a bob since she was little. It has thickened up her fine hair immensely and we always get comments on it - even though there is never a brush through it.
I'm lucky though, she adores going to the hairdresser - just as well as it needs a cut about every month lol.