Reading about the Big 5 in the textbook, I feel it makes sense and I can see how it can be useful in analyzing and predicting people's behaviors. However, I have a reservation about its structure; it measures a person's personality as being high or low in a given trait rather than being more exhibitive of a certain trait versus its opposite. For example, a person with low extraversion may be led to feel he has "less of a personality" in this dimension, as he cannot be described as social and lively. Of course, this is not true at all; he may be any of various degrees of introverted, and it is possible that his personality cannot be justly assessed without measuring just how introverted he is--which would be different from measuring how extraverted he is.
I think this possibly misleading aspect of the Big 5 is important to consider any time it is applied for "real-world behaviors," like predicting success in employment. The textbook states that high conscientiousness, low neuroticism, and high agreeableness are commonly associated with successful job performance. Here, a reasonable inference from the model would be that low conscientiousness, high neuroticism, and low agreeableness often lend themselves to low success in jobs. However, I personally fall under the categories of highly neurotic and lowly agreeable, and yet I've been hired for every job for which I've applied, and I've received raises and awards at my workplaces for commendable performance.
As an alternative to the Big 5, I highly prefer and Myers-Briggs Type Indicator, which measures an individual's preferences in the areas of "favorite world," "information," "decisions," and "structure." I see a deeper value in how this model measures these traits in one direction or another, rather than highness or lowness, and especially how it combines the four results to categorize a person's personality into one of 16 distinctive types. The following site provides further information on the logic behind this model: