We've all heard people decrying the value of IQ tests: particularly that they are either culturally biased, or that they only exhibit one dimension of intelligence.
I suggest additional reasons for that value being limited.
I have always found IQ tests relatively easy - but I'm mathematically inclined. Further, in my experience people that are clearly below an IQ of, say, 80, are clearly lacking some general capabilities.
My thinking is that the upper end of IQ results reflect mathmatical/logical capability, but it doesn't measure the broad range of human capabilities. Likewise, low scores are indicative of a disability. Yet for those who score mid-range, it's hard to say anything useful about their intelligence.
There have been a number of alternatives suggested for the straight IQ measure, such as intelligence that is social, emotional, visual/artistic, musical, and so on. I find myself in agreement that "IQ" measures only a limited range of a person's intellectual capabilities.
My suggestion is that those IQ measures that score mid-range are only demonstrating their mid-range logic capabilities, and that we have no sufficient measure of their capabilites in the broader aspects of human capabilities.
It was suggested to me that those with aptitude for classical music are likely to be pretty intelligent on typical IQ measures. Yet my reading of the music industry more generally suggests that there are many musicians that are neither very logical in general, nor very capable of managing their own lives - even equalising for other factors such as self-medication. Syd Barrett is typically held up for this measure in the music sphere; Van Gogh - and many others - are rightly or wrongly depicted as exemplary in the musical world. I would be surprised if surrogate IQ tests didn't place them mid-range; however, I'm sure there are vast swathes of musicians that are highly intelligence in the IQ measure - it's just that such a measure is not directly relevant to their particular expertise.
[What it has to do with brain function is an interesting question. Recent findings have, for example, suggested that autism is much to do with a differential ability (or dis-) of different regions of the brain to communicate with each other - and that high-functioning autism (so-called savant) may be an aspect of the same, that is, abnormality in the networking of different regions of the brain.]