My DS has taken CoGat twice. He hit the ceiling both times. Personally, I think my dislike for CoGat is how the schools use it. Many schools attempt to use it as an IQ test, which it's publisher clear states it is not a IQ test. It has low ceilings which do not show the difference spectrum between LOG. My DS's GT teachers talk about how they have to "prep" him to take Cogat and other grade level tests. Given him an above grade level and he'll knock it out of the park. Many of these kids over think these test. They try to remind him a lot not to over think it before the test.
Her example was this...
Which one is different an apple, banana, carrot, or kiwi?
My son might say Kiwi because it has 4 letters and the others have 6. He might say banana because it has 3 syllables. Clearly, most kids would say a carrot because it is a veggie VS the others are fruit.

I think Cogat can be fine but not as a stand alone test for identification. Many schools try to use it as their Identifying test for GT programs. It should only be one piece of the puzzle to make decisions.