I'll just add my experience:
I've been buying things from Zavvi for probably two years now, and it's been good. Usually I get a bunch of stuff for myself and my friend and vice versa when things are on sale at a good price. About six months ago, I placed an order that included a 2 for £25 as well as a £5 off £50 coupon code that was posted on their Twitter. I had changed to paying by PayPal since they made the option available so as to have my credit card details on less sites and it was fine up to that point.
On that particular order, they processed half my order, taking the funds from PayPal and sending it, then the other half was stopped with a Payment Problem error. The email they sent me said to check that my credit card had enough funds. I filed a support ticket asking what the problem was, as my bank account clearly had enough money for PayPal to pull funds from, why the email said there was a credit card problem, and what I should do, if anything, to rectify the problem. The response I got ignored my questions and simply said they couldn't do anything about it and I had to place a new order for the items they hadn't shipped.
Right, whatever, I decided I'd save my money and not place a new order. The Payment Problem was never removed from that order and that order was never completed, as their support said they couldn't do that.
The next order I placed, I used PayPal again. The request to access funds showed up fine on PayPal when I placed the order and everything seemed dandy. Next day, same problem happened. I checked my PayPal and they never took the funds from my account, but the entire order was stopped because of Payment Problem. Their support is completely useless and illiterate and doesn't seem to do much more than scan for keywords and post the best canned response they have.
I got my friend to place the order and it went through fine. I haven't used them since, but still place orders through my friend because it seems like my account is basically barred from getting orders processed for whatever reason.