It's a great move to replace Ariza for the Lakers, but I'm not sure adding a 40% shooting, ball-pounding iso player is even an upgrade from Ariza. He does give them a better on-ball defender, but he's going to waste a lot more possessions than Ariza did.
Ariza was such a deadly player for them because he rarely turned the ball over, took a bad shot or made a bad decision on offense, and he frequently created havoc for the opponent on defense.
Artest will knock down a few more 3s (though their playoff %s would suggest otherwise), rough up the team's wing to spare Kobe for offense and take some big balls shots when no one else will, but he'll also jack up some terrible shots, turn the ball over on foolish drive attempts and generally do fewer of the the little things that Ariza did.
Again, far better than replacing Ariza with Walton, but I don't see how it makes them better, just gives them a different twist.