Yes, I suppose one's guests would be surprised - that's one word one could choose - to discover that they've been served a glass of cold condensed soup. But not surprised in a "darling you shouldn't have!" kind of way. No, it's more of a "Your tests results are in. Sit down," kind of a surprise.
And I can guarantee you that the copywriter was snickering as he wrote "you'll be surprised (and so pleasantly) with the wonderful flavor of a Cream of Mushroom Soup Shake." That is something that has never happened in the history of the world. It's like saying "you'll be surprised (and so pleasantly) by the subtle elegance of Martha Raye" or "by the sophisticated policy analysis by Sarah Palin."
Great tumblers, though - gotta give 'em props for that.