Twenty years ago the SZ effects were difficult to measure -- although all-sky surveys were achieving excellent sensitivity to the statistical distribution of MBR noise, it was difficult to measure an arcminute-scale structure with a sensitivity of 100 µK, so the existence of the SZ effect from any cluster was questionable.
Ten years ago the thermal SZ effect was regarded as a proven quantity, with possibilities for providing new and interesting cluster and cosmological information.
Today the SZ effects are seen as important windows into cluster formation physics and cosmology, but they are not yet being used fully because it remains difficult to detect any but the most luminous clusters via their thermal SZ effects, and there is insufficient cluster spectral information to make a statistical measure of cluster kinematic SZ effects.
Over the next decade we will enter the era of precise SZ effect measurements, and their routine use in cosmology and astrophysics. It is now important to design the next generation of instruments, for the new era of SZ effect observations, to match the coming generation of high-sensitivity X-ray satellites.