I've added it to my buttercream before and absolutely hated it. Now, i've been known to eat the stuff right out of the container before, so it's not the taste itself that i hated. It's how it made the texture of my buttercream. It added soooo many air bubble to it that thankfully when i made it, it was gonna be used on a cake covered in kitkats and m+m's. It would've been impossible to smooth. Plus i found it overpowered the icing and made it wayyyy too sweet. Next time i would rather add in some melted chocolate or cocoa powder to make chocolate buttercream. Never again will i add the premade frosting from the store. I believe i had used BC chocolate fudge icing.
On another note...I have a fabulous receipe for a bundt cake and the finishing touch is to melt a can of that ready made icing and pour it over the top of the cake. It hardens slightly..but.. It is heat sensitive and will drip down to a puddle if it's too warm, but otherwise it's a nice easy topping for the bundt. If you are interested i can share the recipe.