Scott,
I think the slabs that the guy has on Albuquerque Craigslist shows that there is potential to the wood. I have limited experience with it, but the grain can be beautiful and it polishes up very nicely.
It is fairly hard, Janka hardness of 2345, compared to Ebony 3220, Honduras Mahogany 2200, hard maple 1450, just for some comparison.
As I understand mesquite is often considered a 'weed' or 'nuisance' tree, grows with a twisted habit that is hard to get much useful wood out of, and thus frequently ends up in the woodpile.
Wikipedia has some information on it.
Bottom line is find out how big the pieces are that your buddy can get access to. maybe he can get some pictures? If they are big enough that they could be useful to you, then it sounds like you have a roadtrip!
Dan