My truck's original belt was changed at about 20K miles along with a belt tensioner under warranty due to noise. The replacement is now at around 100K miles without a single crack that I can see. I have had the fleeting urge to replace it just to have something to do on a Saturday afternoon, but it looks too good. I plan to run it until I see at least one crack.
When I changed the slightly cracked belt on my old truck, I used a Goodyear Gatorback. Absolutely no complaints with that belt, but I certainly can't complain about Motorcraft belts either.