Okay, let me clear this up... N2O is not illegal, so long as you don't tamper with the existing emissions controls. nitrous is street legal, so long as you don't remove any sensors or your cat, you'll be okay. California may be a different story, but I'm speaking generally here. some kits are CARB approved, so just know what you are buying before you shell out the cash.
N2O will add stress to engine components, just as a turbo or SC would. don't overdo it, and you should be okay, only spray when you are racing, or you absolutely need the extra HP.
Also, be sure to use a WOT switch and not a pushbutton for the Nitrous, I can't count how many times some ricer blew his motor because he put his setup on a button. Nitrous should only be engaged at wide-open throttle anyways, so it's a sound investment.