If you already have the HD receiver and you downgrade, you'll really notice.
Before I had HD, I thought my pic was pretty good. Now I'm a bit of an HD snob. I don't watch anything that isn't in HD.
Like a previous poster said, you can probably get local HD with an OTA antenna if you downgrade.
If you are mentioning the $10 HD charge for HD channels, then if you call DTV you should be able to get that fee waived for 24 mos. I called and they gave it to me. I think all you had to be was on autopay. Saved me $240.