Many people I've dealt with claim the Goodyear gatorback serpentine belts slip because of the split cogs.
I've never had an issue with my gatorback belts, and they last me around 50k. I've used Dayco before, they work well, no complaints against them, however to me they're too stiff.
I like the Gates belts, and Gates is the OE supplier to many manufacturers, including AC Delco and Napa.
Weirdly enough, my Saturn Relay has a mitsuboshi belt on it, and while it looks fragile, it seems to still be working. continental is OE to many import mfgr's as well.
I'm not a huge fan of continental timing belts, I've heard of them stretching overtime when they aren't supposed to.
the gates belts are supposed to be heavily nitrile soaked and are supposed to be extremely durable. I personally find them to be a bit to flexible, but I have yet to see any fail due to manufacturing.