A good post from Purely Presbyterian: Isaac Watt’s “Psalm” Imitations
However, I think the term counterfeit is more appropriate. Imitation, especially today, is not regarded as a negative. Those that displace the 150 Psalms with any other song are more guilty than those who pass counterfeit bank notes, and suppress the knowledge of what they are doing. Those passing counterfeit bank notes only seek to defraud men, those using counterfeit songs for worship defraud God (and themselves, depriving themselves of the experience of actually worshiping the LORD). Woe to the ministers and elders that traffic in those counterfeits and take pleasure in it. It could be said regarding the history of the divisions within PCUSA — Old-School/New School, North/South, OPC and PCA, in a manner analogous to God’s assessment of the Kings of Israel: “And he did that which was evil in the sight of the LORD, departing not from the sin of Jeroboam the son of Nebat that made Israel to sin” — And they did that which was evil in the sight of the LORD, departing not from sin of Isaac Watts, that made the church to sin.
Despite his own description Watts was not imitating the Psalms, but counterfeiting them, not to make songs suitable for Christians, but to deprive Christians of the the most intimate way in which the Word of Christ can dwell richly in them, and the sweetest means by which God the Holy Ghost fills them with His Spirit, that is the Spirit of Christ. Despite his own hagiographic projections of his motives, Watts, smooth with guile and subtlety, was leading the churches of Christ away from Word of God with counterfeits, and Watts could have no good motivation. His lie that the Psalms aren’t suitable for Christians in worship, is couched with all the subtlety of and in imitation of the serpent in the garden.
His evil fruit has been the ruin of reformation.