The cx18 driver has its own wiki within the ivtv Project (from which much of cx18's source code was derived). The most current version of the driver is built from the main development repository located here on LinuxTV.
This driver works perfectly for all NTSC/ATSC functions. As of 1/2009, VBI and subtitles are not yet fully functional. As a general recommendation, use a dual-core system to get good ATSC/SD rendering. There are known PCI issues when using older hardware. Older hardware can't smoothly render an ATSC stream anyway...