I have the Sony V series, which is meant to be one of the better ones.
With a decent high quality feed, for example the BBC Planet Earth or whatever it was called looked incredible in SD.
However, give it a low bitrate feed & you can tell the difference! **** in & **** out is basically the rule.
You also need to consider the size of screen you get, & the distance you sit from it. Most people seem to overlook this part & get the biggest screen possible, & wonder why it looks pants at 3 ft viewing distance!