This is probably just me, but my impression is that a cummerbund looks very good if you have a trim waist. But if you're a bit on the portly side, a waistcoat is probably more forgiving. But I'm probably imagining all this.
But there are a few of my friends who wear black tie who think of cummerbunds as a bit fuddy-duddy (though I'm not quite sure why). And I suppose the cummerbundless current Bond doesn't help.