My own opinion of Joel Osteen's message is well expressed by Michael S. Horton in this essay on the Westminster Seminary California website. Link from Monergism.

Although explicit proponents of the so-called "prosperity gospel" may be fewer than their influence suggests, its big names and best-selling authors (T. D. Jakes, Benny Hinn, Joel Osteen, and Joyce Meyer) are purveyors of a pagan worldview with a peculiarly American flavor. It's basically what the sixteenth century German monk turned church reformer Martin Luther called the "theology of glory": How can I climb the ladder and attain the glory here and now that God has actually promised for us after a life of suffering? The contrast is the "theology of the cross": the story of God's merciful descent to us, at great personal cost, a message that the Apostle Paul acknowledged was offensive and "foolish to Greeks."
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Osteen reflects the broader assumption among evangelicals that we are saved by making a decision to have a personal relationship with God. If one's greatest problem is loneliness, the good news is that Jesus is a reliable friend. If the big problem is anxiety, Jesus will calm us down. Jesus is the glue that holds our marriages and families together, gives us purpose for us to strive toward, wisdom for daily life. And there are half-truths in all of these pleas, but they never really bring hearers face to face with their real problem: that they stand naked and ashamed before a holy God and can only be acceptably clothed in his presence by being clothed, head to toe, in Christ's righteousness.

This gospel of "submission," "commitment," "decision," and "having a personal relationship with God" fails to realize, first of all, that everyone has a personal relationship with God already: either as a condemned criminal standing before a righteous judge or as a justified co-heir with Christ and adopted child of the Father. "How can I be right with God?" is no longer a question when my happiness rather than God's holiness is the main issue. My concern is that Joel Osteen is simply the latest in a long line of self-help evangelists who appeal to the native American obsession with pulling ourselves up by our own bootstraps. Salvation is not a matter of divine rescue from the judgment that is coming on the world, but a matter of self-improvement in order to have your best life now.