No test is perfect. My one child who has taken the CogAT did significantly worse on it than on an individual IQ test given a few months prior (around 30 points lower -- bright but not gifted vs. over the 99th percentile). She, however, has also had wildly erratic scores on IQ tests, so I don't know that it was a fault of the CogAT per se.
The main thing that I'd take into consideration with the CogAT is that it is a tightly timed test for grades 3+ and requires the child to sit down with paper and pencil, read the questions himself, and fill it out. It is definitely not a test of creativity. Part of the reason that I suspect my youngest did so poorly on it is the timed aspect and that she is a very divergent thinker. On a test where you have multiple choice answers and only one is "right," there is no wiggle room for interpretation or asking the child to "tell me more" to see if he truly understands the concept like there can be on a verbally administered IQ test.
None the less, some gifted kids do well on the CogAT and it can give a reasonable estimation of ability for some. Even if they test him individually, the CogAT is still a group ability test and it still has the same strengths and faults that it has when administered in a group (save for group distractions).
You may want to take a look at some info on the test from the publisher:
http://www.riverpub.com/products/cogAt/index.html
