I had used them. You only need to create an API key with "deposit" and "getinfo" access rights, therefore there is no chance you're going to lose any funds etc. Just follow their step-by-step guide how to create the key. You are not giving them any credentials this way.
Cancelling is immediate. Then you can (and should) delete the API key you created previously. Easy as a pie.
Whave, I tried to cancel the request. I followed the steps mentioned on the skanner site. But then the amount of the withdrawal to be credit back to my acount was minus the fee ( 1%). Does it mean that Mtgox will charge me for the fee anyway? Is there a way to get back the fee?