In general, the SAT and ACT tests can be useful tools in guessing whether a student will do well in college. The ACT is a slightly different test, in that it focuses on skills learned in high school rather than mere aptitude, but the math and verbal scores do seem to correlate to college success. However, the science and reading sections do not, according to a study published in the "American Economic Journal."
According to a study by Claudia Buchmann at Ohio State University, those who use prep materials, particularly private tutoring, tend to do better on standardized tests. The cost of these prep materials, however, typically approaches $1,000, which makes them out of reach for all but the wealthiest students. Additionally, students who can afford to do so may re-take the SAT and ACT several times, and colleges will only use the best scores. This means that those who can only afford to take the test once might not have as many opportunities to shine.
According to an interview with Jon Katzman, founder of The Princeton Review, on PBS Frontline, the SAT test is primarily a test of test-taking skills rather than actual knowledge. Some students simply aren't good test takers. They might need more time to complete the questions, which might earn them a lower score, or they might simply be bad at guessing the right answer. This does not necessarily mean that they'll do poorly in college, though, as a high score doesn't necessarily guarantee success.
Fortunately, college admissions officers realize that standardized test scores are not the be-all-and-end-all of a student's potential. They use the scores as part of the whole picture. If a student has a high GPA and a strong essay, for example, the officer might write off a low standardized test score as just a bad day. Likewise, a high standardized test score coupled with a poorly written essay or low GPA might also set off some alarm bells.