Have never been to Zaxby's before, but went there with some friends today and of course had to try the sweet tea (my favorite soft beverage). Good tea is pretty important to me so I keep close tabs on restaurants that have good tea, since it's what I drink when I'm out. For those of you that have never been, I thought I'd offer my take on it.
The tea itself was a nice color, with no bits of "tea debris" floating in it - definitely a good sign. The taste was excellent. There was no "charred flavor" from steeping it too long, and the sugar content was just right. It was sweet enough to be sweet tea, but not too sugary to overpower the spiciness of the tea itself. I suspect less black tea was used because there was absolutely no metallic taste whatsoever. A very clean, crisp flavor that tasted equally well in all parts of the mouth.
As far as the ice used, Zaxby's provided what is by far the best ice in my opinion. This is the small, pellet-shaped ice cube a little bigger than a pencil eraser. It is just large enough to retain its shape (not melting and thus watering down your tea) but small enough not to form an "ice berg" and leave one part of the glass frozen and the other parts lukewarm. For a typical 8-ounce glass I recommend a third of a glass of ice filled to the top with tea. This will give you enough ice to flash chill your tea but leave it warm enough to enjoy the flavors, not necessitating the quick swallowing of frozen tea. For larger glasses, you'll need 1/3 - 1/2 cup of ice, but keep in mind that you'll need to drink the tea slightly quicker to prevent dilution of the ice.
Just my two cents, as I'm enjoying a very invigorating buzz from 2 glasses of tea