Justice for All gives you more of what the first Phoenix Wright game did; if you like the first one, you'll like this.
Playing the first game is not a requirement for playing this one, but it helps to understand some things such as references to old cases and knowledge of the characters. The new psyche lock system is fairly straightforward, you can sometimes get people to tell you things they're hiding or lying about by presenting evidence or profiles in a dialog with them. Wrong accusations will damage your health bar; it will however refill quite a bit upon a successful "unlocking."
This game is easily worth getting if you're a fan of the first. If you haven't played the first, it's a rather unique game and depending on your tastes you may or may not like it.