That said, I struggle at times with his writings on Christian Hedonism (and not because of the common "we're-called-to-deny-not-gratify" objection, which I don't think is a valid critique of his position). In short, Christian Hedonism is the philosophy that "God is most glorified in us when we are most satisfied in Him." It is rooted in the notion that we are not only allowed, but in fact called, to enjoy God, and that doing so in no way detracts from His glory or makes Him a means to an end. So far so good.
In this regard, Piper's works like "Desiring God"  and "When I Don't Desire God"  are very helpful for spurring the believer on to greater joy in God and to a passion for his glory. Christian Hedonism is wonderful news when seen as a promise ("Yes - you CAN find ultimate joy in God rather than sin, and his glory and your joy are not at odds"). However, when pushed beyond a simple promise of joy in God, I've found that Christian Hedonism strikes me as very bad news - as threat, rather than promise.
Whether Piper intends it or not (and I am confident he does not intend it), it can be a subtle move from "You can (and ought to) enjoy God" to "You'd better start enjoying God - or else!". This can shift our focus from "all that God is for us in Christ" (the true source of our joy in Him), to what I do for God (i.e., enjoying Him sufficiently as a condition of my salvation). None other than C. S. Lewis, one of the inspirations for Piper's Christian Hedonism, noted that "the surest way of spoiling a pleasure [is] to start examining your satisfaction" (p. 218, Surprised By Joy ) .
I think the challenges of consistent Christian Hedonism in the Christian life come to forefront in Piper's book "Future Grace" . In particular, in Ch. 15, his discussion of the nature of saving faith seems to me (despite all Piper's protestations to the contrary) to confuse the fruits of faith with the nature of faith. He states:
"I don't want to say merely that faith in promises produces 'confidence, joy and hope,' but that an essential element in the faith itself is confidence and joy and hope[...] The essence of saving faith is a spiritual apprehension or tasting of spiritual beauty, which is delight... [In sum,] if we do not taste the beauty of Christ in his promises as delightful, or as satisfying, we do not yet believe in a saving, transforming way" (pp. 205-206, emphasis original).
My beef with this is the focus on the fruit of faith (confidence, joy, hope, delight, and satisfaction) as faith itself. I understand Piper's position that saving faith is a gift of God (a point with which I agree). And I also appreciate his desire to guard against "cheap grace" and "easy-believism." But I'm not convinced that Scripture presents faith primarily as a satisfaction in God (particularly satisfaction as interpreted through Piper's strongly emotional lens). Rather, it seems to me that faith consist most fundamentally in trusting God to keep His promises, irrespective of our ability to contribute anything (including our affections and delight) (e.g., Rom. 4:16-25, Heb. 11).
If we were to adopt Piper's conceptualization of faith as delight, then it seems we'd need to place an asterisk next to many of the key statements about faith or belief (Greek ) in Scripture. For example:
15 “The time has come,” he said. “The kingdom of God has come near. Repent and believe the good news*!”
*i.e., "Repent and be satisfied with all the God is for you in Christ!"
He then brought them out and asked, “Sirs, what must I do to be saved?”
They replied, “Believe* in the Lord Jesus, and you will be saved—you and your household.”
* i.e., "Delight in the Lord Jesus"
The difficulties with the faith-as-delight view are especially striking in Paul's epistles:
16 know that a person is not justified by the works of the law, but by faith* in Jesus Christ. So we, too, have put our faith in* Christ Jesus that we may be justified by faith in* Christ and not by the works of the law, because by the works of the law no one will be justified.
9 and be found in him, not having a righteousness of my own that comes from the law, but that which is through faith* in Christ—the righteousness that comes from God on the basis of faith*.
* i.e., joy
At this point, I ask: If God, the Author and Lord of language, who had any words at His disposal to express the truths of salvation, intended to drive home the point Piper is making, why did He choose to express it primarily in terms of faith, rather than of delight? Under Piper's definition, it appears we are not justified by faith, but by satisfaction/delight/treasuring/joy. I know that Piper would not deny justification by faith - in fact, he is one of the most ardent defenders of the doctrine in our day. However, I worry that the above formulation of faith ends up denying justification by faith indirectly, by redefining faith as something other than trusting God's promises. If he objects to N.T. Wright's supposed view of justification "on the basis of/according to works," how does Piper avoid asserting a justification on the basis of/according to affections?
I realize this all sounds very critical of Christian Hedonism, and I don't intend any disrespect to Piper, a far greater man than I am. I just wonder how it is that we're supposed to be continually delighting in and treasuring God when, under Piper's system, I'm continually in danger of proving to have never been converted if I don't continually keep up a sufficient level of delight in God (How much in order to know I'm saved? Just a little more than I've got now?). If I wake up on the wrong side of the bed and don't feel that "zip" in my morning devotion, this may signal that I'm reprobate. And am I supposed to enjoy relating to the God who is about to damn me to hell for my lack of enjoyment of Him? I don't see how!
In short, if I must believe in order to be saved, and I must delight in order to "really" believe, how can I delight until I know that Christ has actually accomplished salvation for me? It seems little grounds for joy to say, "Only when you get adequately satisfied in this Christ who might not yet be your Savior, will he prove to be your Savior - so get on with delighting, you could-be-saint!"
On the other hand, when I contemplate the objective work of Christ on my behalf (rather than my subjective work of delighting in Him), I find this brings real joy! I think I'll go read some Luther...
 Piper, J. (1986). Desiring God: Meditations of a Christian Hedonist. Sisters, OR: Multnomah.
 Piper, J. (2004). When I don't desire God. Wheaton, IL: Crossway.
 Lewis, C. S. (1955). Surprised by joy. New York: Harvest.
 Piper, J. (1995). Future grace. Sisters, OR: Multnohmah.