Gonna stick up a little for SD here.....although my comments are based on having a decent upscaling TV and are aimed at higher bit-rate channels....
Whilst obviously, on Freeview, which is main viewing platform, you've got BBC1/2/ITV/CH4 HD, their SD counterparts are not at all bad - perfectly watchable on my new Sony 55" W9 set... ok - why would you watch SD when the same content is on HD - true - of course - but SD can sometimes look very good indeed depending on the source and equipment used to upscale.
Some of the channels like Dave or ITV3 etc don't tend to look as good, but they generally seem to be broadcasting on a lower bit-rate so the SD images here will look poorer on a huge LCD/LED/Plasma screen, but overall, SD can be quite ok - I watch a lot of older archive TV stuff (as well as BBC4 and BBC3 regularly) and I dont have many complaints regarding the quality of the image on this SD material..... certainly not on my TV anyhow!