Accuracy? My personal score seemed close to reality as I am generally happy, though not all the time. I usually have a positive attitude, and its usually under times of stress, exhaustion, and misfortune that my attitude turns the other way.
Change? The questionnaire asks you to answer the questions based on the last few weeks/months. I feel like it doesn't give as accurate of a representation of someone's happiness as questions regarding the last year or two could.
Bias? The questions, the way they were worded, were somewhat vague, and as its already difficult to judge yourself in an inventory, the broadness made it hard and my answers generalized.