I hear this argument a lot and I still don't buy it. I use more advanced functionality than 90% of my coleagues and I don't find google spreadsheet stops me in any way.
Tell me something excel can do that Google spreadsheet can't.
If you're using excel as a programming interface it's going to be hard to dig out of that. Of course, one could argue that excel was never a good place for that sort of thing in the first place.
Tell me something excel can do that Google spreadsheet can't.