Young Justice isn't quite up to the standard of Justice League Unlimited. It lacks that show's Iron Age political consciousness. But truthfully, that's the absolute worst thing you could say of it.
Like all the best superhero cartoons, this show is as not just a series, but a serial. It's a show where a large scale story arcs are gradually built up over the course of a whole season - and indeed, not only within each season, but between the seasons. I say "like all the best", because that's the format that's always seemed best able to bring out the more adult side of the superhero genre. It not only allows for more sophisticated plot lines: It also allows for the gradual development of character, and the relationships between the characters, while still leaving plenty of space for the spectacular action sequences that all fans of the genre love.
I know this is a controversial topic among fans, but personally I like season two a little better than season one. However, due to the complex nature of this show, I would say that seeing season one first is an absolute must.
I'd give the show itself (and yes, specifically season 1) 5 stars no problem. However, I'm taking off a star due to the dodgy nature of the DVD release. This business of releasing shows in "volumes" rather than seasons is just so typical of the way things work in animation. Come on, Warner Bros.! You don't do this with your "grown up" shows!