It's a wonderful game, I love it. Have you every heard of Phoenix Wright? It's a lawyer DS game where you use your evidence and facts to point out that the one in question is lying and thus bring out the truth. Like, they say they fell in the water at 7:43 pm but their stopped watch shows 6:24.
This is an element I quite enjoy that is in L.A. Noire. But just an element, just enough to make your brain tingle.
I'm sure you know that they used real actors to animate all important faces in the game as well? This enables you to actually SEE if they're lying or telling the truth. And it works. The graphics aren't perfect, but the animation, acting and realism in it really makes it worthwhile.
The game lets you drive around in "a perfect re-creation of 1947 L.A.", chase criminals on rooftops, making you Spider-Man. You're in shoot-outs, you search crime scenes... oh, it's great. Do get it, if you like such things.
P.S.: the musical score is also great, with smooth jazz and music from that period.