Recommended for ages 15+ (I'm 18 and found it to be enjoyable, as well as even moderately thought-provoking at times)
So to begin, I watched the original uncut Japanese version of the show. I have not seen the dub (I was able to sit through about 60 seconds of it to the point when Yuusei said, "I'm coming for you Jack, and this time I'm going to settle the score"). While the subbed version is (supposedly) darker, the characters are much more positive role models. i.e. in the first episode Yuusei is not trying to 'settle the score' with Jack (which honestly sounds rather violent to me), but simply wants Jack to return the card he stole and possibly make up with him (what a concept!). The show focuses on people working to get along with each other. The main characters show compassion to their 'enemies' and people who are not accepted by the rest of society. They also seem to genuinely care for the safety and well-being of others. (as a note, Jack, who is extremely arrogant at the beginning of the series goes through a lot of positive character development, I've read this is not as drastic in the English version).
Another positive aspect of this show is that it is moderately thought-provoking. It raises questions such as: How do our actions affect others? Is it right for some people to live in poverty while others are rich? It also presents the idea that good people are born into unfortunate circumstances and can't simply 'pull themselves up by the bootstraps.' One of the main messages of this show (in my mind) was that everyone should have a chance to have a good life (there shouldn't need to be a 'Satellite' to complement the 'City').
The possibly negative: the show is somewhat violent, characters get hit/kicked, they also injure themselves in motorcycle accidents, some characters die, the cards fight during the card games. (I would suppose the majority of this does not happen in the English dub). Some characters have sad/violent back-stories (several grew up in poverty). There is also theft. The language they use is not particularly bad, but it's not always polite (watch it for yourself and decide). The Japanese itself is certainly not formal super polite Japanese, but I don't recall every hearing anything particularly awful. These are also scenes where people are drinking, however I do not recall the main characters drinking at any point.
While in my opinion this show is not the best thing for your younger child to be watching, it is unlikely anything in this show will have a serious negative impact on them.
Another plus to having your kids watch the Japanese (subbed) version is that even though the show does not have any concrete educational value the fact that it is in a foreign language and created in another country will at the very least allow them to pick up some of the language and culture. The subtitles will also force them to read. (As a warning, if you are thinking of watching this to learn conversational Japanese, please don't. This show is fine if you want to work on your comprehension, but please don't address yourself as 'waga' or 'ore' like Jack).
The Japanese (with subtitles!) and English versions are both available on Hulu for no cost.
What other families should know
Great role models
Too much violence
Too much consumerism