Great information from the AVS forum here showing how one cable company in particular degrades their HD offerings. Its like trying to fit 10 pounds of potatoes in a 3 pound bag, somthings got to give, and in this case its picture quality. What does this have to do with off air antennas you ask?
In many markets, the only way to get uncompressed HD is from over the air, most local DTV brodcasters send out their HD feed with no or minimal compression. Best of all, HD from an antenna requires no monthly charge.
While this poster compares Cable to Verizon FIOS ( A great service) the point is how much most multi-channel providers must reduce image quality to fit HD and the the rest of all of their programming and services onto limited bandwidth.