yes the subchannels usually look like poo, but they're also usually reruns of bad 70s tv shows and aren't ever going to look great.
(though they may be fun to watch regardless)
I find the quality of the sub channels varies. Some of the shows on PBS are clearly filmed with Hi Def cameras, and even though the broadcast is at 480i (SD), the quality is a bit nicer than many of the old TV shows. Unfortunately, our newest modern techology, the LCD screen, does not handle SD very well. Some brands are better than others, but most people, seeing it for the first time are a bit dissapointed. That's why TV stores never have an SD signal showing on the display units. They wouldn't sell many if they did. Plasma handles SD a little better, but the truth is the old tube Tvs did a much better job of it.
With the proper compression, it's actually possible to have three 720p HD signals broadcast in the bandwidth available to each local OTA affiliate. But very few are doing that. NBC and CBS both insist that their signals be rebroadcast at 1080i, so that only leaves SD for any other offerings in the sub channels. Fox uses 720p for their main feed, and I think ABC is flexible, so there is opportunity there for more HD in the sub channels.
LG 42" 1080, Sony S550 Blu Ray, Pioneer VSX 512S, Harmony One, Roku 3-wired, Fire TV Stick, FlircUSB, 30Mb, Antenna. Coventry RI, USA