Up to you, personally, I've used the OEM Bando belt just fine, though the Goodyear Gatorback works much better (no hint of chirping or squeaking). Cost wise, it was a few dollars difference. I sprung for the better (quieter) belt.
Little cracks are normal, but if you have a large number of deep, wide cracks or chunks of the belt are missing and/or the back of the belt is showing the inner core (threads showing) - then it is time to replace that belt. Normally, I shoot for changing the belt at 60K miles or so - the old belt generally looks OK, I end up tossing that in the trunk as a backup belt, incase the one on it breaks for some reason.
Here some good pics - not mine, I just Googled them up:
The little, fine cracks are normal. The heavy, wide cracks toward the left side of the pic, spanning four adjacent ribs - that is getting close to replacement time.
Another one, those little cracks are OK. Note the deeper ones go through the depth of the rib and into the belt core - that is a sign of a belt that is done.
Same applies to the poly-cog ones - the belts that have individual cogs or links, cracks will form in the troughs of the belt, but any missing cogs is grounds for belt replacement.
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