I've noticed the difference on a regular 1080p TV. Cable often compresses the hell out of broadcasts so they can squeeze in more channels.
I was thinking more of the sharpness of the image than macroblocking / compression artifacts when I made my comment. Frankly, a decent quality DVD upscaled using madVR's NGU algorithm on my HTPC looks closer to (or perhaps surpasses) OTA / cable HD in sharpness than OTA / Cable HD does to a 1080p Blu-ray.
For example, I watched a bit of MTV Live yesterday. It's a "HD" channel. I'm pretty sure DVD playback on my HTPC is sharper and looks crisper than that channel. I think generally OTA HD is better looking than the HD on cable. With few exceptions neither is particularly sharp. Perhaps ironically the local news is some of the sharpest looking OTA content since it only gets compressed once.
It's all a trade-off. Low'ish bitrate, good sharpness, and (limited) macroblocking. Pick the two you want. You can't have all three from a realtime MPEG-2 encoder.
