Video quality is fine. It's probably never looked this good so you won't have any problems there.
The only point of note is that some (just a few) scenes have audio sync problems where the sound isn't completely perfectly in time with the picture. There aren't many though (I'm up to Season 4 and have only struck a handful) so it's not a show-stopper.
I've experienced this a few times with other discs and switching everything off and then on again fixes it, so those occurrences are probably due to some connectivity sync problem. Doing the reboot doesn't fix these Northern Exposure sync issues though - they're on the disc itself.
The good news is that they appear to be confined to just the scene when you notice them. When there's a cut and a new scene follows, the audio is back in sync.
While audio sync problems drive me nuts, they're pretty minor on this set and I like the series enough to be able to overlook it.