How reliable/accurate are otc digital ovulation tests and monitors such as the clearblue fertility monitor?

OPK: They reliably detect urine lh, but less good for predicting ovulation. Since lh is nature's trigger for ovulation once a follicle is mature, we assume ovulation follows in the next days. This is true in textbook cycles, but is variable in real life. Ironically, they work best with clockwork cycles, and less well for women who are less predictable.