My SO asked me this last night, and I stumbled through the answer, mostly because I just assumed we all knew it was a bad thing. Putting aside the obvious (taste), I mentioned the high caloric content and preservative modifications made to it in processing. She was unimpressed.
This is the result of my drive to stop buying HFCS products and improve the food quality in the house. Anyone have a great source of info on the stuff that will help me help her?