My experience adding frozen concentrated apple juice has been good. It concentrates the apple flavor. But you can over-do it. I once added eight 12-oz frozen concentrates to a 1 gallon carboy, then topped it with apple juice. The initial SG was 1.15, almost as thick as maple syrup. When it finished fermenting it was over 18% ABV, but still sweet and super apple-tasting. It's too sweet for me, and I blend it down with a more dry cider before drinking it.
Recent discovery: At the grocery store Sprouts they sell an un-filtered apple juice in a 1 gallon clear carboy. You can ferment right in the container it comes in. I measured specific gravity and it was 1.050. I poured off just over 12 ounces, then added one 12-oz frozen concentrate apple juice, straight in, no added water. It brought the SG up to 1.060. Then I pitched in ale yeast, Coopers from diybeer.com.au and also I added a dozen little pieces of oak that I'd toasted in the toaster oven for 90 minutes.
Anyhow, you don't have to buy carboys at the brewing store. They cost me $8 each, and this one gallon bottle from Sprouts cost the same, $8 but came filled with unfiltered, & very cloudy apple cider, ready for fermenting. A #6 rubber stopper fits the bottle.