I discovered this series back in 2001 when ADV was first releasing the DVDs. Back then, I was waiting for Disney to release "Castle in the Sky". I heard that "Nadia" had a lot of similarities to Miyazaki's film, which is what drew me to it in the first place.
All I can say is that it is a great series... for the most part anyway. The characters are likeable and interesting (Jean, in particular, is my favorite), and the story setup is engrossing and very imaginative... until you get to episodes 23-34. At that point, "Nadia" as a series takes a complete 180 degree turn for the worse. The animation declines drastically, the characters turn into caricatures of themselves (Nadia in particular devolves from an interesting heroine into a totally unlikeable, bizarre b***h), and the concepts are absolutely devoid of the complexity and depth the opening eight episodes and the "Nautilus" chapters shared. The only exception out of this is Episode 31, which fills in background information on the protagonist's past. But "Nadia" never comes back until the final five episodes (35-39), and even then, while the ending is indeed exceptional, its impact would be much stronger if the bad episodes were excised from the series. (I later found out that these episodes were added on to cash in on the shows popularity with a different director at the helm, which explains everything.)
It's too bad that "Nadia" suffers from such an unfortunate flaw, because its best parts are some of the best in Anime. Some of my favorite episodes include:
Eps. 1-8: The beginning of the series. Especially the Gargoyle's island arc (5-8). That is arguably the best "arc" in the whole show.
Episodes 14-19: These are my favorite "Nautilus" episodes--all of them are superbly written, edited, directed, and characterized. The ones which involve exploration and marvels (16, 18, 19) and especially traumatizing situations (14, 15), in particular, are superb, and what makes "Nadia" as a series.
Episodes 21-22: An awesome closure to the "Nautilus" arc
Episode 31 (and parts of ep. 30): If there's any part of the Island arc that is resolutely watchable, it's this segment.
Episodes 35-39: Must-sees, no questions asked.
-Jon T.