I work for Ford Credit. If you co-sign for someone, you co-guarantee the note. That means that you and the Buyer are both equally responsible for the payment, and the credit history on the account is reported equally on both your credit report and the Buyer's.
Give this some real thought. If the Buyer decides he/she cannot pay or if he/she should receive negative credit reporting on the account because a payment goes 30 days past due, then you are on the hook as if YOU bought the car. The lien holder (whoever that will be) will have the right to go after both parties for payments in arrears.
Co-signing also affects your ability to borrow. Since a creditor will know that you *could* become responsible for the payment on the car in the future, this fact will be taken into account when you apply for credit.
I have seen train wrecks as a result. An example is a girlfriend wants a car but does not have the credit to obtain it. You do. You co-sign. You then have a verbal agreement that she will pay the payments. You break-up. She decides that she cannot or will not pay any longer. The account goes past due 30 days, then 60, then 90. Then a repo occurs. This is reflected on both of your credit reports. I see this every day.