I'm usually hesitant to give 5 star ratings, but Boston Legal truly deserves to be noted for its excellent quality. The top-notch acting and playful humor that grace scenes within the office distinguish it from any other court drama I've ever seen, and likely any show that has been on television for some time. That being said, a lot of the humor requires either a considerable number of years under your belt, or an extensive knowledge of your parents' (maybe grandparents') stories. As such, as the years roll on, this show will sadly lose much of it's appeal.
I personally think the show has positive messages, but if your politics differ from mine, I'm sure you'll disagree in some fashion. Beware the lecherous men of the show: it's clear to a shrewd viewer that this is presented as a character flaw, but a kid or younger teen may not. That being said, I doubt anyone of that age range would find the men of the show particularly appealing to model themselves after. At some point in the first season, Denny and Allan, the main characters, begin smoking a cigar and drinking scotch together at the end of each episode. They typically have a charming little discussion that serves as a capstone on the episode, but if you find the use of cigars and alcohol as social affair objectionable, keep this in mind.
What other families should know
Too much sex
Too much drinking/drugs/smoking