There is really no secret in Comfort’s, “best kept secret”. Preach the Law and repentance before you present grace is standard fodder among those in the Reformed tradition. But among the revivalists and easy-believe advocates, getting a person lost before they get them saved is truly a missing piece of the gospel message. Comfort rightly rejects a man-centered gospel, which may fill churches but that doesn’t necessarily mean a soul has been brought to Christ.
Beyond these positive points, Hell’s Best Kept Secret has little to offer. It was similar in tone to most manuals on witnessing, attempting to excite the reader about the task, mainly through examples of cold-turkey evangelism. Of a more serious nature is Comfort’s Arminian theology and obvious Pentecostal leanings. He is a big fan of Charles Finney while at the same time quotes Charles Spurgeon. He believes in power evangelism, the Pentecostal second blessing, and speaking in tongues, although he does not make these things central to his message.
Comfort has a good thesis but it is not worth wading through all the other errors and misuse of the Scripture to get there.
ADDENDUM: In personal correspondence (August 2008), Ray Comfort has informed me that he has renounced his past Armenian and Pentecostal leanings. For an understanding of his present views see his more recent publications.