While not as common, PBS productions are usually good. NOVA is typically excellent -- historical documentaries can be hit or miss, but Secrets of the Dead is typically pretty good, and History Detectives can be fun.
I'll take a PBS documentary over any of the cable channels' productions, any day of the week -- but there's not as many of them. :-(
Sometimes the cable channels surprise me, for example Mike Loades latest documentary, although that wasn't an American production, and it was really really condensed.