Like Byrdman said, if there's lates associated with the co-signed account, both co-signors will feel the affects of those lates (meaning, whacked score).
The BIG deal in co-signing is that you've made that debt YOUR responsibility. If the other co-signor (ostensibly, the one who's actually going to pay for the debt) decides to say "Blank my bills!" YOU are then on the hook. Not just credit-reporting wise, but LEGALLY.
If a debt defaults, the creditor WILL sue you. And you will be liable. Completely.