Go for it, particularly if you can get 'em for free.
I think the biggest question is how long they are, as too big they will be an encumbrance and if cut down much you will lose the curve and twist...there is always the question of how thick, are they brittle, etc., common to all horns, but you will have to get the core out before you can answer those questions, I think.
I believe that a lot of the F&I War horns with that lovely twist were made from Spanish cattle, shipped up from the Caribbean, and therefore made from the ancestors of the longhorns.
I think it is definitely worth picking out some with a nice shape and giving them a go. I've been eying the longhorns on a "ranch" near me (longhorns in the Blue Ridge, go figure...) and wondering if it would be worth it to go talk to the owner.
If nothing else, a longhorn would make a nice storage horn, I bet. Also make good fodder for making rum horns and the like.