ISEE scores have different weights in the application process depending on the school. Different schools have different expectations on ISEE scores. Your ISEE score is not the only factor in the independent school application process. A high score on the ISEE along with good grades, extracurricular activities, recommendations and other parts of your application will make your application hard to turn down. However a high score alone may not be able to offset a weak academic profile.
In short, a good score is relative.
Why is the ISEE important? How is the ISEE structured?