Observations of the instability wave growth in a series of continuously operated low and high aspect-ratio inclined settling experiments are reported. The effect of wave rollover and breakup on the efficiency of operation of such devices is also examined through measurements of the contamination of the clarified overflow product. The general trends of the stability and efficiency in low aspect-ratio settlers are found to be consistent with those set forth by the linear stability analysis of Borhan and Acrivos [Phys. Fluids 31, 3488 (1988)]. In particular, settler stability and efficiency are enhanced with increasing suspension concentration and fluid viscosity. It is also shown that high aspect-ratio inclined settlers not only provide the largest enhancement in the settling rate but are also more stable and efficient than their low aspect-ratio counterparts.