I've been using one for quite a while now. I've tried both synthetic and down ... the synthetic one is rated to -5 and packs to about half the size of a 'normal' synthetic bag with the same rating and tips the scales at 1kg. My down quilt is a Go-lite Ultra 20 which is rated to -7(although I'm slightly sceptical), weighs just over 550g and packs to the size of an average decent down jacket.
Besides the obvious weight/bulk benefits there's also the fact that they really suit 'side sleepers'. I find it's quite easy to get tangled up in a normal bag and often wake with the hood above me ... must be all the tossing and turning

If anyone from AlpKits's listening, can we have a PD600 quilt please, ta.