In one way its useful. I'm definitely an INTP, and I love it. The results really made me happy about myself. I really like personality tests.

On the other hand, it's not perfect. Human personalities are very complex, and it's hard to pin it down. I've had the struggle of finding my result. Once, I was an INFP. Then, I was an INTJ. After a while, I took the quiz again and gave my best answer. I got INTP.

In conclusion, I think its just for fun.