Its blisteringly fast! Mapserver is quite fast but serving the cached tiles make it very quick. “So long as you can run python CGI, and write to disk, you can cache any WMS for your own use”. It has support for WMS and also TMS (Tile Mapping Service) requests.
Currently they are serving their Vmap0, Blue Marble, Human Footprint and USGS Digitial Raster Graphic datasets.