I watch a lot of documentary programming, and all the channels -- A&E, Discovery, History, TLC, Court TV, and so on -- can be hit or miss. They all feature a lot of "reality TV" and things like ghost stories, "psychic detectives" and the search for Bigfoot that lead me to change the channel.
Most of their programming is good, entertaining, factual and somewhat informative, but not particularly in-depth or insightful. Most of the Bill Kurtis ouevre falls in that category for me. Maybe 10% is really gripping and educational.
I haven't observed that any one channel is consistently better than the others, except that PBS, which has far fewer hours of documentaries than the networks that carry them 24 hours, has consistently more depth and higher production values. To what degree that's offset by bias or agenda is an argument I'll leave aside for now.
At least I know I'm not missing much -- thanks!