Well, I'm temporarily... on Dish, Adelphia cable, and VOOM.
SD on Dish is pretty bad, mainly due to a soft picture, and sometimes smearing images - but only rare pixelization.
SD on Adelphia (on our "fiber" system) is pretty good, even the analog stations. The digital tier is pretty good except the movies services SD is superb, no ... outstanding.
Okay, SD on VOOM - varies. Spike can be fairly pixelated (a fine type in the background - almost a grain). Some other channels like TBS and esp. the SD movies are quite good. But NOTE: I have found that the SD looks MUCH better if one uses either 1080i or 480p with component feeds. Frankly, the 480i images are not that great. Otherwise, I have to say the images have more sharpness than Dish, and after seeing SD on VOOM, Dish SD looks downright out of focus or something. I prefer the sharper images of VOOM over Dish, even with some pixelization.