For me, the 600/6.3GM is a must have. Not meant to replace the 600/4GM at all (opposite to quite a few others I am not looking to replace the 600GM with e.g. the 300GM and TC's)
But meant to be a (very) lightweight and small alternative for hikes and cycletours when I am not really looking to engage in photography, but want 600mm at hand when something special does show up.
