Highly underestimated, if you ask me. This show incompasses a few of the best actors on television, like Sam Watterston.
There's no ongoing plotline- a new case is presented every night (or week, depending on what channel you watch.) The first half of the show is generally concentrated on the crime and the detectives who investigate it. After lots of complicated police work, and generally a lot of frustration, the "Order" part commences.
I don't have much to say about the men who play the police officers. Lenny, the main detective, is completely detached from his work, and therefore very hard to relate to. Unless you're really interested in the methods police go about to catch criminals, it's more fun to just tune in half an hour after the show has started.
Jack McCoy is the main lawyer, played by Sam Watterston. The best part about the show is the almost complete lack of character relationship. Jack has no torrid love affair with his partner, Claire. They remain on a platonic level, which makes the show entirely more tolerable. The characters are well developed without being constantly in some out-of-work crisis.
Jack is famous for his summations, near the end of the show. I have, honestly, been moved to tears by some of the points this character has made in cases. He is impassioned, and throws himself into every case, no matter how detached he is from the situation. Jack was a protester hippie guy in high school, and his constant search for real justice is apparent.
The good guy doesn't always win. The plausibility factor is high throughout the show- while some of the cases are not realistic, all of the courtroom scenes are. There is high intensity, yes, but no melodrama as seen in "Ally McBeal" or "The Practice." If you can handle disappointment and gruesome crime scenes, Law & Order is an incredibly intelligent and well written show.