As far as an improvement in SD channels, I don't think so. I didn't have any issues before, though.
Could be just my particular display is needy of it. Or (I can’t remember what screen size your running) I noticed a consensus that some who jump to a progressive DVD player on a smaller high-quality CRT screen do not notice a huge improvement. (or not large enough for the dollars). It seems to be the large signal hungry screens that realize a net noticeable improvement. (Trash blown up is BAD).

I see the same as you between programming (HD feed signal quality). Some HD I’ve run into has been riveting, some no better than a high quality DVD recording played through a progressive player.
When you get that ‘blend’ best quality HD feed on your HD capable/integrated screen it can pop out at you. Looking forward to slowly seeing more and more of it. The other day I ran into a football game on 13.2 (?) here. It was so beautiful and lifelike so, down (on and in) the field I had to watch it for awhile, even though the game did not interest me.

Turner F. First we purchased a HD RCA-DTC100. I believe just (or about) to be discontinued so it might be found for a good price. Once we had the 60” inch enlarged trash problem on our hands - It dramatically improved SD for us over the original Direct TV provided box, before we even switched out the single round dish for the dual HD dish. However the DTC100 was SLOW to aquire channel change lock and menu descriptions) We then switched to the Sony HD200, again just before the dish itself was switched out a couple of weeks ago. Again PQ improved on SD material. (Slightly) Channel switch and movie details screen load amazing faster than the DTC100. Still, the HD200 has my eyebrow raised for 30 days. On the AVS boards, the HD200 has had some digital audio dropouts to a point of being unusable for some since its launch. Some of this behavior was traced to the bit-stream signal from certain stations, and having e-mails flying to station managers around the county, when the signals were changed the problems were reduced. Some seem connected to a sensitive and faulty (resistor) I believe. (that issue was supposedly addressed). Still something in the Sony seems sensitive to audio signal loss in a way that older HD models did not exhibit such as Hughs. The other issue with the HD200 has been reboots. (which wipes your favorites channels) and interrupts viewing as the 200 takes about 1 to 2 min. to rescan all channels on a reboot.

We’ve had the reboot twice but no digital audio dropouts. It might be interesting to note that some conjectured on the AVS boards that certain prepro/receiver brands and models (Sony, Denon, B&K Ref50) might have conflict with the HD200 creating some of the digital dropouts. Paired with my 950/770 we have had NO audio dropouts. Because of the signal hungry nature of my display which really requires the HD boxes quality to make SD watchable and the fact it has DVI, which the HD200 also sports (and my husband wants the use of that connection). We may keep the Sony, but the jury’s still out here.
Yes it’s a better PQ than the older model DTC100. But the unit has some issues.
If your doing a search on boxes on shelves be aware that the Zenith HD520 is the same platform as the Sony HD200 (both made by LG electronics). You won’t trade off any issues (just how you feel about choosing which CS you get stuck with and 100 bill less for the Zenith) by choosing between those 2 models.
Go to AVS forum to the HDTV hardware section, here:
http://www.avsforum.com/avs-vb/forumdisplay.php?s=38b5750e524f6df1015a2745d9847433&forumid=25
you will find many user reviews on the current models and a nice stuck header by AVS listing new models with what specs are confirmed with estimated releases in the next year.