The Hardshells are divided on the issue of whether the use of musical instruments is allowed in new testament church worship, as are the Campbellites (aka "church of Christ"). Those who oppose the practice think that those who have them are sinning against God in having them, and cannot be a legitimate church with them. My dear friend, Bob L. Ross of Pilgrim Publications, wrote a book on this in which he completely overthrows the views of the Hardshells and Campbellites. I encourage all to get his book "Campbellites, Cowbells, Rosary Beads, and Snake-Handling." (see here for many segments of the book) He also has had several debates with the Campbellites on this topic, one of which can be seen on Youtube. All speeches are lined up here and I encourage all to listen.
One of the famous books on the topic was between the two segments of the "Church of Christ," and represents a debate that was had on the topic between N.B. Hardeman and Ira Boswell. To read a pdf file for the Hardeman-Boswell debate on the subject see here. To me Bob completely destroys the reasoning of the anti-instrumental music crowd.