Taken from an Amazon.com Review:
The most glaring problem occurs in the episode "Day of the Dot" (the classic Big Pete and Ellen kiss on the football field) -- a noticeable and slightly annoying tape error creates a constant line of red in the upper section of the screen, and it stays that way for about five to ten minutes.
But other than that, there's really not much else to complain about.
Nickelodeon obviously didn't know how to handle this show properly; if there are any other faults noticeable on these episodes, it's just how they are now. There's no way to go back and reverse the damage done to the master tapes throughout the years.
That's not to say the quality is God-awful; far from that. If you have seen the recent and much lamented Noggin re-runs, you'll know what you're getting. Neither above-average or below-average quality, just good enough.