DeepDiscount.com has really good customer service. I've contacted them a few times, and always been pleased. It might seem that if I've had to contact them multiple times it would imply that their service sucks, but I'm sure that by this point I've placed a hundred or two orders with them, so a few mistakes is to be expected. In the past, they've replaced broken CD cases, and sent me a new copy of one of the seasons of Star Trek: Voyager on DVD months after my order, as I hadn't gotten to that point in the series yet to see that my disc was scratched. Most recently, they let me exchange my full-screen Indiana Jones DVD set for a widescreen one and still keep the 20% discount I got when I first placed the order. I didn't even ask for anything special; I just wrote "meant to order widescreen" on the invoice when I sent it back. It's my favorite store on the internet.
I've also had very good luck with Comcast's customer service. I've called them a few times for various reasons, and it's always been fast and pleasant. My call to downgrade my cable service from $40 basic cable to $10 limited cable took all of a couple minutes, with no pressure or badgering. Last time I called them it was because I got a past due notice for all of my cable service since moving here. I was on auto-pay at my old apartment, and my payment information didn't survive the transitions, so I really hadn't been paying them. Within like three minutes I was off the phone, with my bill paid, autopay reinstated, and late fees removed from my account, all without having to plead for anything.
I had a very different experience with art.com. I ordered a fairly expensive large framed print from them a while back, and when it arrived, the plastic cover keeping it safe from the outside world... was taped to the frame. The frame had a nice gold foil coating, except for that one tape-shaped black spot. So, I emailed them to get return authorization. They sent me, via UPS, a shipping label. (This is kind of silly, since you can just print a UPS shipping label, but whatever.) They also promised that they had paid for the pickup, but when I called UPS to schedule it, they assured me that art.com had not paid for anything; they just paid for the label to be delivered to me. So, I had to pay $11 to return my item. They didn't want to send a replacement until they received my returned item, which is somewhat reasonable since it was a large custom-made picture, but they also didn't want to apply the coupon code that they sent me in my first order's receipt. I explained that I could just as easily get a full refund for my initial purchase price, and place a new order to be shipped immediately using the new coupon code, but they didn't get it. Finally, I was sick of them and just demanded that they cancel my order and get a refund instead of a replacement. Well, apparently "cancel" was the magic word, because from that point forward I was emailing a new customer service rep, who was happy to adjust the total on my order. In the end I got what I paid for, but a couple weeks late, with a whole lot of hassle, and I still haven't gotten my $11 back for the UPS fees. In the end they got what they paid for too: a customer who's placed a half dozen orders with them that is quite unlikely to ever order from art.com or allposters.com again.
So, the moral is: spend lots of money at deepdiscount.com, and avoid art.com.