I strongly recommend that you DO NOT CANCEL.
First, while cancelling means your scores do not go out, Pearson warns that when you take the test again, the schools which receive your scores will be advised that you had previously taken a test, and requested not to send out your scores, which just plain looks bad.
Second, to be honest, when I finished the test I felt really bad about my Quant section, and so I gritted my teeth when I had to click "accept" on the scores. I was very pleasantly surprised by the 730 I earned.
I have a suspicion that if, like me, you try to get every answer right, at the end of the test you will be dissatified and just a little bit worried about your scores, but you will get good scores. I have read several reports from people who were happy with their test until they saw the score, and several who were unhappy with how they did until they saw the score, so feeling tense and exhausted at the end of the test may actually be a good sign
Hopefully other test-takers will tell us how they felt.