I've used it for 3 of my larger apple purchases as well to get the interest free financing, but it's a pain after the promo period is over - Barclays will fight you and tell you that you don't have an active interest free promotion.
The first time this happened, I noticed my purchase was accruing interest, so I called Barclay's Customer Service to find out what happened. They fixed it for me and credited back all the interest I was charged and applied the 18 month period after the fact, as it had taken me a couple months to catch what happened. They then told me that if I was wanted to make a purchase using that promo, to call first so they could activate the interest free option.
So the next time I bought something I did just that, and then was told I couldn't have the interest free promo at all. I about had a fit. What was the point of getting the credit line? Why would I use it if I could finance something interest free through AT&T? I had to push for it, but they gave it to me since I had been told I could keep using it like that, but they said I had to make my purchase first, then call, then they added the promo. That was a few months ago.
Then I had to buy new phones a month later, but I still had a large balance due to a recent iPad purchase, and I didn't feel like dealing with the hassle, and AT&T had a BOGO going, so I just did the AT&T monthly payment thing. Not sure if I will ever try to use the Apple card for interest free again since it apparently isn't what they make it out to be, and was truly annoying to jump through the hoops, and deal with stupid customer service.