I think it's a great pickup, I know AI and Melo may have a luv 'em or hate 'em appeal. But the talent can't be questioned.
I mean trying to stop both of them will be a chore and both are able to take over a playoff series. As far as defense, AI will playing the passing lanes, always has always will. Melo is a great spot up shooter extending to the 3 point line. He doesn't need to dominate the ball to score, he really is a quick strike type player (love that about him.) AI will penetrate at will, Melo will spot and slash, ti could be great.
They key will be finding a role playing good defender at the other guard spot (I think JR Smith, may need to be a six man.) Camby on the interior and a willingness to play team defense to a passable degree and they are fine. The Key IMO is the strong garbage man defender who can also hit the three pointer (i.e. Bruce Bowen, Battier, Posey etc...) Battier would've been great there, as he is anywhere.
The one thing the Nuggets failed to do was throw Nene in the deal for Steven Hunter, that would have been great but even the Sixers aren't that stupid.