Location: Vancouver - OTA, Pioneer KURO Elite Signature PRO-141FD, 27" iMac, WDTV Live.
I have all seasons of 30 Rock in HD from iTunes. The quality is great. There's not a lot of action or anything in the scenes so macro-blocking is never an issue. If I were to compare it to the quality of the OTA and Cable broadcast equivalent, it's definitely on par.
In fact, all of the HD shows I've downloaded from iTunes for ATV are exceptionally nice looking.
Echo the remarks by Cyclism. I haven't rented 30 Rock (television about television is off my list), but I've been watching shows I wanted to sample to see if I liked them e.g. The Wire.
Both SD and HD have been flawless, even look good on my 24' BenQ in the kitchen. Pleasantly surprised.