I used a promotional coupon for Adwords and received £75 worth of free advertising- which is, of course great...
However you are not informed when your credit has run out and you have to register a direct debit account when you sign up- this is instantly debited for whatever the amount may be once the credit is used up.
The costs per click add up VERY quickly and you begin to incur quite a big bill before you can cancel.
Not only are you not contacted or warned about the mounting costs, their actual billing page is VERY confusing, even if you are internet savvy, it appears that you have credit when you do not.
Please keep a sharp eye on your account page when you use their coupon and don't expect any communication or constructive replies from Google.