yes, I was billed twice for an something (app or music track, can't remember). I contacted them and they refunded both amounts, no hassle. If you go into your account and then purchases you can raise queries over things there.
Someone hacked my account and used up about £40 I had left in there from a voucher I received.
I went to my account to raise a querie, but it said as it was an 'in app' purchase they were not responsible for refunds and to contact the app author. I then emailed apple direct, within an hour I had a reply and all the money was refunded and my account password reset.
Couldn't fault the service I received from the person who handled my problem.