I did TENG very much, and have the two-DVD set myself. (Haven't seen all the special features yet, but I'm working on it.) It manages to break the Disney stereotypes of the young "diamond in the rough" type, the mentor, the love interest, etc. Instead, they concentrate on four GREAT characters, and settle in to just be funny for 80 minutes. It never pretends to be epic, it's just darn fun.
As for why it sank in the box office... I think part of it comes from Disney's lack of support for it (or even really clarifying what it is, as a lot of people I know were turned off by David Spade going "uh-huh, uh-huh" after he rescues Pacha from the cliff... that scene was in EVERY commercial). See, Disney is sick of this film. As you've seen in the DVD set, it started out as the VERY different "Kingdom in the Sun." What the DVD may not tell you is that the flick was very much a stereotypical Disney flick with the songs and the diamond in the rough and all that. And it was so bad, that the head Disney honchos said that, even after lots of money was spent and lots of animation was FINISHED, the producers need to take this in a different direction or it'll be cancelled. Hence the new, funny direction. Problem is, all the Disney execs were so sick of dealing with the movie that they didn't realize that it wasn't just adequate, it was darn good. Hence the weak push.
Which is a shame, I'd love to see Disney do an all-out comedy every once in a while for variety. Still, at least it looks like they're going to a straight adventure flick with Atlantis... I'm really excited about that one (and no songs, either).
Personally, I'd compare TENG to a feature-length Looney Tune. Check it out.