I'm not in favour of having medals no matter what - mostly they are cheap rubbish 
Of the medals that I have kept (very few), the Amsterdam half marathon is pretty good - shaped like the running track at the Olympic stadium (where it finishes), and not too big.
Many races seem to think bigger is better (eg. London Marathon).
I've also kept a few from French races. The best British medal I've had was for the Burnham Beeches half a few years ago.
Edited: 21/11/2012 at 12:53