GMOS is the first telescope - spectrograph combination that acts as a complete system to deliver enhanced image quality and stability while simultaneously exploiting the large aperture of an 8m telescope. The entire system (optics, mechanics, software, detectors) was designed to take advantage of the best images that the Gemini telescopes produce while being extremely reliable and efficient. The built-in wavefront sensor allows the telescope to quickly point at an object, optimize its focus and then track it precisely for many hours (possibly over several nights) while maintaining perfect telescope and instrument focus and providing first order image compensation. As a result of the carefully-engineered design of its structure and mechanisms and its active flexure control system, GMOS offers unique scientific opportunities. A recent enhancement was the implementation of the "nod and shuffle" technique to give improved sky subtraction for very faint object spectroscopy. Some of the scientific highlights of GMOS' many modes (Imaging, MOS, IFU, precision velocities) are reviewed, and some of the "lessons-learned" during the first few years of operation are described.