Paul Helm offers a gentle criticism of Piper's "Christian Hedonism" here.
I think one problem with Piper's approach is that it does not deal with piety covenantally. Whereas Scripture, and the Reformed confessions, teach that piety is born out of the gratitude that is created in the heart and mind of the redeemed sinner by virtue of being made right with God, Piper proposes that piety is born out of an "appreciation" of the excellencies of God that is not so much mediated through the preaching of the Word and the Sacraments but through an immediate, predominantly emotional, awareness of God's perfections.
The Heidelberg Catechism states:
Question 86. Since then we are delivered from our misery, merely of grace, through Christ, without any merit of ours, why must we still do good works?
Answer: Because Christ, having redeemed and delivered us by his blood, also renews us by his Holy Spirit, after his own image; that so we may testify, by the whole of our conduct, our gratitude to God for his blessings, and that he may be praised by us; also, that every one may be assured in himself of his faith, by the fruits thereof; and that, by our godly conversation others may be gained to Christ.
The neo-Platonism is as evident in Piper as it is in his hero, Jonathan Edwards.