I have to admit I am not usually a huge fan of buttercream frostings, even when they are dairy. I tend to prefer things like ganache and cream cheese frostings which pack more flavor with less sweetness. However, some kinds of cakes really do need the sweet fluffiness of a good buttercream icing. When that is the case nothing else will do. I have experimented on and off for years trying to come up with a parve buttercream worth eating. Since buttercream is primarily butter and sugar, substituting margarine tends to create a strange greasy mouthfeel that makes me not want to eat more than a bite, if even that. Shortening is even worse (think of what the big colored roses on cheap bakery cakes tastes like).
Chocolate Buttercream Icing