The Lowell Observatory Instrumentation System (LOIS) is an instrument control software system with a common interface that can control a variety of instruments. Its user interface includes GUI-based, scripted, and remote program control interfaces, and supports operational paradigms ranging from traditional direct observer interaction to fully automated operation. Currently LOIS controls a total of ten instruments built at Lowell Observatory (including one for SOFIA), NASA Ames Research Center, MIT (for Magellan), and Boston University. Together, these instruments include optical and near-IR imaging, spectroscopic, and polarimetric capability. This paper reviews the actual design of LOIS in comparison to its original design requirements and implementation approaches, and evaluates its strengths and weaknesses relative to operational performance, user interaction and feedback, and extensibility to new instruments.