A persons IQ only measures their intelligence with respect to the average. It compares your score to the rest of the population, and assigns you a percent value (50% is average).
For comparison purposes, a persons IQ is probably the best way to measure intelligence, but it doesn't put an absolute value on how "smart" a person is. To score well on an IQ test, a person doesn't need to know much more than basic mathematics and English An IQ test is more about the skills you have for applying your brain power.
IQ is quite reliable as far as it goes, in the same sense that your time on a 100 meter sprint is a quite reliable metric for how fast you can run a short distance.
Disagreements on IQ are more on whether it actually matters much. The cartoon Dilbert had a strip where Dilbert visits Mensa (an organization for people in the top 10% of the IQ scale) and wonders why they aren't all rich and famous. The reply is along the lines of "IQ turns out to have very few practical applications".