Not the most helpful work. Flavel here undertakes an admittedly difficult task: the distinguishing of the hypocrite from the true believer. However, as Flavel himself admits, there is no litmus test for true belief. Any one 'sign' of faith could very well be present in a mere pretender. Likewise, any 'symptom' of hypocrisy could manifest in a god-fearing saint. Indeed, honest Christians will readily admit that the symptoms of hypocrisy are far more prevalent in their lives than they should be.
Flavel is thus left with the following conclusions:
Conclusion 1: X is a clear indicator of hypocrisy, except when it isn't.
Conclusion 2: Y is a clear indicator of grace, except when it isn't.
A theologically sound set of statements, to be sure, but perhaps not terribly helpful. Particularly since the insecure Christian, reading about the signs of hypocrisy, will in all likelihood be quick to identify them in his own life and become even more insecure. And he may not be wrong--there is something of the hypocrite in all believers (a predicament that is unlikely to change until we are gathered to heaven). Meanwhile, the foolishly confident 'professor', oblivious to the signs of hypocrisy in his life, may be able to point instead to 'signs' of grace which further bolster his otherwise unfounded assurance of his faith.
I realize that Flavel's intention was precisely the opposite--that is, he intended to reassure to the doubting Christian and to rebuke the hypocrite--and this work may well have had the intended effect on many readers. However, I found it confusing and unsettling.
That being said, if you suspend the ultimate conclusion--that is, the answer to the question 'Am I really a Christian?'--the text could be read as a moderately helpful tool in identifying areas for growth. Thus, instead of frantically wondering whether the presence of X sign in your life makes you 'not a real Christian', simply accept that sign as sin, praise the Lord that it has been paid for on the cross, ask God to enable you to fight that sin, and start fighting. So taken, as a treatise on the sin of hypocrisy, the book could be useful. It's still not up to quality of Flavel's other works--The Mystery of Providence and A Saint Indeed are my favorites thus far--but it'll do.