I don't think its a good predictor at all. I know many people who graduated from high school and college with high GPAs (3.8+) who are complete and total morons. It all depends upon the quality of the school, and the classes you take.
One of my HS valedictorians is super-smart. She's a math genius. Got something like a 34 on her ACT.
The other one took the easiest classes possible, took short day, 4 years of P.E., but she had a 4.0
I was salutatorian, because I got a B in a gifted Algebra II class I took as a soph (its a jr. level class). She got an A in a regular Algebra II class she took as a senior. Yeah, its real fair that she was ranked higher than me (and a host of other people who'd worked really hard in much harder classes)
My school district doesn't have weighted grades for honors/AP/gifted classes because some woman complained that her "little darling" wasn't valedictorian when she had a 4.0. The reason was becasue other people had worked their butts off and her kid was taking regular level classes.
IMO, standardized tests are a much better indicator of intellect. They present a standard, and test how you measure up to that standard. Because there's such a vast difference in the quality of school districts around the country, its hard to judge by grades. A 4.0 is not the same everywhere.