Plase keep in perspective that I am NOT a die-hard Superman fan, so I'm not familiar with the history of the character.
I would have never watched this show if it wasn't for Batman The Animated Series, but you must know, that everything coming from Bruce Timm & CO is a must watch. The first part from "The Last Son Of Krypton" and you're hooked with it. Voices are the same high quality as the entire DCAU. Tim Daly pulls an amazing Superman, Dana is THE perfect Lois Lane, Clancy Brown IS Lex Luthor for me, and the list goes on and on and on.
Animation is great. While it was not as thin and cheap looking as "The New Batman Adventures" (don't get me wrong, that animation looked great) it wasn't as good as the animation in Batman Animated Series. While Batman TAS had the best animation, there were some episodes that had HORRIBLE animation, but with Superman this doesn't happen. While the best animation went for the first bunch of episodes and the most important ones during the show, it never hit a low point of bad animation, it was always great, consistent and with very few imperfections.
My only complaint about the show (and the reason I'm not givin' it a 10 rating) was that the quality in terms of story telling and specialy, character development went a little bit downhill as the show progressed. In the first season of the show, although all the episodes had their own villain and story, it had a season long arc about Proffesor Hamilton investingating Kryptonian tecnology, such as the Phantom Zone projector, Superman's ship, etc; all of this dissapeared from Season 2 until the end of the show. In fact, Hamilton doesn't appear again until the last episode of the show. Lex Luthor (THE superman enemy) was in almost every episode of Season 1 (not as the main villain) and then appeared in some of Season 2 and almost nothing until the last episode. The show forgot about developing characters and started making new stories with new villains or new heroes that had no influence on the show's continuity.
When all of this happened, villains that had only an episode in the show, returned, and were ruined. Some of them are: Toyman (from the excellent "Fun and Games" to the lame "Obssesion"), Mala and Jax-Ur (from the great "Blast From The Past" to the middly entertaining "Absolute Power") and Mr. Mxyzptlk (from the hilarious "Mxyzpixilated" to lame team-up episode "Little Big Head",that also ruined Bizarro).
This is why I like Volume 1 better tan 2 or 3, because all the episodes are connected. Volume 2 and 3 have episodes that stand out from the rest (the great "The Late Mr Kent", "Apokolips Now", etc) but Volume 1 is more consistent and I enjoy it a little bit more.
But there was a series-long arc. The Darkseid story is one of the best things of this show. Starting in small cameos, to indirect villains to mind-controlling Superman, the Darkseid arc kept you wanting for more in every episode and I liked them all (except maybe, "Father's Day", although it had a nice end, with the meeting of Superman and Darkseid).
Overall, this show is amazing. It doesn't have the same impact on me as the Batman shows, but it's really nice to watch and have fun at any time, and get away from the moody Batman for once!