Crab apples are great for making hard apple cider. I make
alot of my own cider and wish I had access to more crabs.
according to Claude Joliceur in The New Cider Makers Handbook (a great book for getting into cider making, though kinda spendy) crabs have several things going for them:
1: Tannins, this is what makes them bitter/sour. Most table apples here in the states are very low in Tannins, which contribute to the mouthfeel and complexity of a Cider (or wine).
2: Sugar, Crab apples actually have higher percentages of sugar than normal apples (we just don't taste it because of the tannins), this leads to a stronger finished product both in terms of alcohol by volume and just overall flavor.
However, rarely can you make an excellent cider out of only one variety of apple and so, as mentioned by leila, I'd blend them with some other types.
Hard Cider is actually pretty easy to make, with endless nuances to make a better product.
I'm guessing that these are
native or naturalized trees? If they are on your property, or a property that doesn't mind you tinkering, It's completely doable to cut back the tree and then graft a more desirable variety of apple onto them. This could help you get a better blend for even better cider (or less fun uses for apples like eating and the like).
Hope that's helpful.
Josiah