We describe a novel instrument dedicated to making rapid angular-dispersive grazing incidence X-ray reflectivity measurements. A novel, automatic, optical technique for rapid specimen alignment, is incorporated into the control software. We discuss the information content of diffuse scattering data collected in non-standard modes. Examples of data are presented showing the application to the characterization of semiconductors and metal multilayers. The technique is shown to be particularly powerful for measurement of the thickness of epitaxial films of AlGaAs on GaAs less than 50 nm thick and where high resolution X-ray diffraction becomes impracticable. We demonstrate that, as the method is insensitive to dislocation density, high quality data can be taken rapidly from heavily relaxed multilayers. Minimum criteria for adequate information content in the data are explored and the effect of specimen curvature is examined.