I love the GK Termies, I don't care if they were rubbish they would still get a place in my army cos they look so lovely and shiny.
However, I think that it should always be kept in mind that some of the reasons they are very good such as ignoring Inv saves with Pyscannons and Incinerators only really come into their own when playing Daemons. Against Imp Guard for example - they are overkill and normal termies would be ust as effective.
Yes I know you can fry Eldar Farseers and warlocks. but everthing in a Daemon army is Inv saves. Imagine how scared they are of incinerators.