Not so bad, really...
In comparison to Buffy and Smallville, both of which were targeted towards teenagers, the Justice League of America really wasn't that bad. Sure, the action was cheesy, the costumes were dorky, and the characters weren't faithful - but neither were Buffy or Smallville, and both fared pretty well in television.
I actually like the idea of a TV show about a down-to-earth group of super heroes who battle super villains while struggling with their daily lives. The only thing the writers should have done differently is created their own original super heroes for the show, instead of having it loosely based off the Justice League.
Kind of like NBC's "Heroes", now that I think about it.